<System.Runtime.InteropServices.DllImportAttribute("gdi32.dll", SetLastError:=True)> Private Function CreateRoundRectRgn(ByVal X1 As Int32, ByVal Y1 As Int32, ByVal X2 As Int32, ByVal Y2 As Int32, ByVal X3 As Int32, ByVal Y3 As Int32) As IntPtr
    End Function
    <System.Runtime.InteropServices.DllImportAttribute("gdi32.dll", SetLastError:=True)> Private Function DeleteObject(ByVal hObject As IntPtr) As Int32
    End Function
    <System.Runtime.InteropServices.DllImportAttribute("user32.dll", SetLastError:=True)> Private Function SetWindowRgn(ByVal hwnd As IntPtr, ByVal hRgn As IntPtr, ByVal bRedraw As Boolean) As Int32
    End Function
    Public Sub RoundCorners(ByRef oFrm As System.Windows.Forms.Form, Optional ByVal Angle As System.Byte = 15)
        With oFrm
            Dim lRet As IntPtr = CreateRoundRectRgn(0, 0, .Width, .Height, Angle, Angle)
            SetWindowRgn(.Handle, lRet, True)
            DeleteObject(lRet)
        End With
    End Sub

function CreateRoundRectRgn(
  nLeftRect,
  nTopRect,
  nRightRect,
  nBottomRect,
  nWidthEllipse,
  nHeightEllipse : integer
): HRGN; stdcall; external 'gdi32.dll';

procedure TFormX.FormPaint(Sender: TObject);
var RRR : HRGN;
begin
  RRR := CreateRoundRectRgn(0, 0, Width, Height, 45, 45);
  try
    SetWindowRgn(handle, RRR, true);
  finally
    DeleteObject(RRR);
  end;
end;

  
procedure TForm1.FormResize(Sender: TObject); 
var aHrgn: HRGN; 
begin 
  aHrgn := CreateRoundRectRgn(0, 0, width, height, 50, 50); 
  if aHrgn > 0 then 
    try 
      SetWindowRgn(Handle, aHrgn, true); 
    finally 
      DeleteObject(aHrgn); 
    end; 
end;  

Option Explicit
Private Const cPointToPixel = 1.333333
Private Declare Function CreateRoundRectRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long, ByVal X3 As Long, ByVal Y3 As Long) As Long
Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long
Private Declare Function SetWindowRgn Lib "user32" (ByVal hwnd As Long, ByVal hRgn As Long, ByVal bRedraw As Boolean) As Long
Private Declare Function FindWindowA Lib "user32" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long

Public Sub RoundCorners(ByRef oFrm As UserForm, lWidth As Long, lHeight As Long, Optional ByVal Angle As Byte = 15)
Dim lRet As Long
Dim LHwnd As Long
    With oFrm
        LHwnd = FindWindowA("Thunder" & IIf(Application.Version Like "8*", "X", "D") & "Frame", .Caption)
'       on travaille en  pixels
        lWidth = lWidth * cPointToPixel
        lHeight = lHeight * cPointToPixel
'        découpe
        lRet = CreateRoundRectRgn(0, 0, lWidth, lHeight, Angle, Angle)
        Call SetWindowRgn(LHwnd, lRet, True)
        Call DeleteObject(lRet)
    End With
End Sub