Hi sambath,

Refer below link. You need to replace the email sending code with your sms sending code.

Send email to all users which the checkbox is checked in ASP.Net GridView

Finally i found. thanks for your hints

        protected void btnsend_Click(object sender, EventArgs e)
        {          
            int count = 0;
            foreach (GridViewRow row in gridsendsms.Rows)
            {                               
                CheckBox CheckBox1 = row.FindControl("chkSelect") as CheckBox;
                //Checking for the checkbox is checked or not
                if (CheckBox1.Checked == true)
                {
                    string Name = row.Cells[2].Text;
                    string Number1 = row.Cells[3].Text;
                    string Message = "Dear " + Name + "! " + TextBox1.Text;
                    //**************** Send Message *******************************************
                    string URL = "http://pay4sms.in/sendsms/?token=snstoken&sender=SAM&message=" + Message + "&number=" + Number1 + "";
                    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(URL);
                    HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
                    StreamReader sr = new StreamReader(resp.GetResponseStream());
                    string results = sr.ReadToEnd();
                    sr.Close();
                    count++;                    
                    Label1.Text = "Sucessfully sent " + count + " Messages.";
                    mpealert.Show();
                }
            }            
        }