Try:

loginForm.StartPosition = FormStartPosition.CenterParent;
loginForm.ShowDialog(this);

Of course the child for will now be a blocking form (dialog) of the parent window, if that isn't desired then just replace ShowDialog with Show..

loginForm.Show(this);

You will still need to specify the StartPosition though.

Assuming your code is running inside your parent form, then something like this is probably what you're looking for:

loginForm = new SubLogin();
loginForm.StartPosition = FormStartPosition.CenterParent
loginForm.Show(this);

For the record, there's also a Form.CenterToParent() function, if you need to center it after creation for whatever reason too.

When you want to use a non-blocking window (show() instead of showDialog()), this not work:

//not work with .Show(this) but only with .ShowDialog(this)
loginForm.StartPosition = FormStartPosition.CenterParent;
loginForm.Show(this);

In this case, you can use this code to center child form before display the form:

//this = the parent
frmDownloadPercent frm = new frmDownloadPercent();
frm.Show(this); //this = the parent form
//here the tips
frm.Top = this.Top + ((this.Height / 2) - (frm.Height / 2));
frm.Left = this.Left + ((this.Width / 2) - (frm.Width / 2));

It works in all cases, swap Form1 for your main form.

Popup popup = new Popup();
popup.StartPosition = System.Windows.Forms.FormStartPosition.Manual;
popup.Location = new System.Drawing.Point((Form1.ActiveForm.Location.X + Form1.ActiveForm.Width / 2) - (popup.Width / 2),(Form1.ActiveForm.Location.Y + Form1.ActiveForm.Height / 2) - (popup.Height / 2));
popup.Show(Form1.ActiveForm);

On the SubLogin Form I would expose a SetLocation method so that you can set it from your parent form:

public class SubLogin : Form
{
   public void SetLocation(Point p)
   {
      this.Location = p;
   }
} 

Then, from your main form:

loginForm = new SubLogin();   
Point p = //do math to get point
loginForm.SetLocation(p);
loginForm.Show();

If any windows form(child form) is been opened from a new thread of Main window(parent form) then its not possible to hold the sub window to the center of main window hence we need to fix the position of the sub window manually by means of X and Y co-ordinates.

In the properties of Subwindow change the "StartPosition" to be "Manual"

code in main window

private void SomeFunction()
{
    Thread m_Thread = new Thread(LoadingUIForm);
    m_Thread.Start();
    OtherParallelFunction();
    m_Thread.Abort();
}

private void LoadingUIForm()
{
    m_LoadingWindow = new LoadingForm(this);
    m_LoadingWindow.ShowDialog();
}

code in subwindow for defining its own position by means of parent current position as well as size

public LoadingForm(Control m_Parent)
{
   InitializeComponent();
   this.Location = new Point( m_Parent.Location.X+(m_Parent.Size.Width/2)-(this.Size.Width/2),
                              m_Parent.Location.Y+(m_Parent.Size.Height/2)-(this.Size.Height/2)
                            );
}

Here the co-ordinates of center of parent is calculated as well as the subwindow is kept exactly at the center of the parent by calculating its own center by (this.height/2) and (this.width/2) this function can be further taken for parent relocated events also.

As a sub form i think it's not gonna Start in the middle of the parent form until you Show it as a Dialog. .......... Form2.ShowDialog();

i was about to make About Form. and this is perfect that's i am searching for. and untill you close the About_form you cant Touch/click anythings of parents Form once you Click for About_Form (in my case) .Coz its Showing as Dialog

The parent probably isn't yet set when you are trying to access it.

Try this:

loginForm = new SubLogin();
loginForm.Show(this);
loginForm.CenterToParent()

    protected override void OnLoad(EventArgs e) {
        base.OnLoad(e);

        CenterToParent();
    }