Form fc = Application.OpenForms["폼이름"];

출처 : http://stackoverflow.com/questions/2576156/winforms-how-can-i-make-messagebox-appear-centered-on-mainform

// 클래스에 사용되는 네임스페이스들

using System;

using System.Text;

using System.Drawing;

using System.Windows.Forms;

using System.Runtime.InteropServices;

//1단계 - 파일에 아래와 같이 클래스를 추가한다 - 시작

class CenterWinDialog : IDisposable {

    private int mTries = 0;

    private Form mOwner;

    public CenterWinDialog(Form owner) {

        mOwner = owner;

        owner.BeginInvoke(new MethodInvoker(findDialog));

    }

    private void findDialog() {

        // Enumerate windows to find the message box

        if (mTries < 0) return;

        EnumThreadWndProc callback = new EnumThreadWndProc(checkWindow);

        if (EnumThreadWindows(GetCurrentThreadId(), callback, IntPtr.Zero)) {

            if (++mTries < 10) mOwner.BeginInvoke(new MethodInvoker(findDialog));

        }

    }

    private bool checkWindow(IntPtr hWnd, IntPtr lp) {

        // Checks if <hWnd> is a dialog

        StringBuilder sb = new StringBuilder(260);

        GetClassName(hWnd, sb, sb.Capacity);

        if (sb.ToString() != "#32770") return true;

        // Got it

        Rectangle frmRect = new Rectangle(mOwner.Location, mOwner.Size);

        RECT dlgRect;

        GetWindowRect(hWnd, out dlgRect);

        MoveWindow(hWnd,

            frmRect.Left + (frmRect.Width - dlgRect.Right + dlgRect.Left) / 2,

            frmRect.Top + (frmRect.Height - dlgRect.Bottom + dlgRect.Top) / 2,

            dlgRect.Right - dlgRect.Left,

            dlgRect.Bottom - dlgRect.Top, true);

        return false;

    }

    public void Dispose() {

        mTries = -1;

    }

    // P/Invoke declarations

    private delegate bool EnumThreadWndProc(IntPtr hWnd, IntPtr lp);

    [DllImport("user32.dll")]

    private static extern bool EnumThreadWindows(int tid, EnumThreadWndProc callback, IntPtr lp);

    [DllImport("kernel32.dll")]

    private static extern int GetCurrentThreadId();

    [DllImport("user32.dll")]

    private static extern int GetClassName(IntPtr hWnd, StringBuilder buffer, int buflen);

    [DllImport("user32.dll")]

    private static extern bool GetWindowRect(IntPtr hWnd, out RECT rc);

    [DllImport("user32.dll")]

    private static extern bool MoveWindow(IntPtr hWnd, int x, int y, int w, int h, bool repaint);

    private struct RECT { public int Left; public int Top; public int Right; public int Bottom; }

}

//1단계 - 파일에 아래와 같이 클래스를 추가한다 - 끝

// 테스트 메시지 박스를 쓰는 방법

// this 가 부모 form 임

private void button1_Click(object sender, EventArgs e) {

    using (new CenterWinDialog(this)) {

        MessageBox.Show("Nobugz waz here");

    }

}

// fmain -> 부모 Form

// 부모 Form의 좌표, 크기를 계산

int mainformx = fmain.Location.X;

int mainformy = fmain.Location.Y;

int mainformwidth = fmain.Size.Width;

int mainformheight = fmain.Size.Height;

// fchild -> 자식 Form

// 자식 Form의 크기를 계산

form fchild = new form();

int childformwidth = fchild.Size.Width;

int childformheight = fchild.Size.Height;

// 자식 Form 보이기

fchild.Show();

// 자식 Form의 위치 수정

fchild.Location = new Point(mainformx + (mainformwidth / 2) - (childformwidth / 2), mainformy + (mainformheight / 2) - (childformheight / 2));