Can not load RxJs

I use this import statement in one of component file:

import {BehaviorSubject} from 'rxjs';
@Injectable()
export class UserService {
    private currentUser = new BehaviorSubject<User>(null);
}

When the browser loads the app, it throws the exception below:

SyntaxError: Unexpected token <
Evaluating http://localhost:3000/rxjs
Error loading http://localhost:3000/app/components/app.js
at eval (native)
at SystemJSLoader.__exec (http://localhost:3000/vendors.js:3320:16)

I don't know why it didn't work. Cause Angular2 worked already. Could any one please help me this case?