try
ScriptManager.RegisterStartupScript(this, GetType(), "alertMessage", "alert('Test Message');", true);
ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alert", "alert('my message');", true);
use
single quotes
for
alert message string
string s = "Notification : Your Message.";
ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "Notify", "alert('" + s + "');", true);