i've upgraded my vb.net application from visual studio 2005 to 2013 and and when i click to show a form which its windowstate is set to maximize , the form shows in design size and i need to open another form and go back to it to get it in maximize size
the main form of application is mdi container

There must be some in your program that overrides your setting. You can try this at the form load event:

 Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.WindowState = FormWindowState.Maximized
    End Sub