>ok la~~~ thank you , but if i want to base on parent form change size? ...
>How to current location - before location ?
>
>

>Private Sub MainForm_SizeChanged(ByVal sender As Object, ByVal e As EventArgs)
>        Dim f As Form = DirectCast(sender, Form)
>        ChildForm.Width = f.Width / 2                          <?
>        ChildForm.Height = f.Height / 2                      <?
>        'Or whatever
>    End Sub
>
>

Not sure I understand what you want. If you want to keep the child form proportional then maybe something like:

ublic Class Form1
    Private ChildForm As Form
    Private OldSize As Size

    Public Sub New()
        InitializeComponent()
        OldSize = Me.Size
        ChildForm = New ChildForm()
        ChildForm.MdiParent = Me
        ChildForm.Width = 200
        ChildForm.Height = 200
        ChildForm.Show()
     End Sub

    Private Sub MainForm_SizeChanged(sender As Object, e As EventArgs) Handles Me.SizeChanged
        Dim f As Form = DirectCast(sender, Form)
        ChildForm.Size = New Size(ChildForm.Width + Me.Width - OldSize.Width, ChildForm.Height + Me.Height - OldSize.Height)
        OldSize = Me.Size
    End Sub
End Class