PHP无法读取文本框值

I'm creating a basic website for class, On my login html page i have 2 textboxes Username and Password and a Login button. On submit i call the php file. It reads the txt_Username but doesn't read txt_Password. Do you have to read a password textbox differently?

Thanks in advance.

HTML page code;

</div>
<div id="Content">
 <form id="loginForm" name="loginForm" method="GET" action="Login.php">
  <table width="27%" border="0" align="center">
  <tr>
  <td width="42%">Username:</td>
      <td width="58%"><input type="text" name="txt_Username" id="txt_Username" /></td>
    </tr>
    <tr>
      <td width="42%">Password:</td>
      <td width="58%"><input type="password" name="txt_Password" id="txt_Password" />    </td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td>&nbsp;</td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td><input type="submit" name="btn_validate" id="btn_validate" value="Login" /></td>
    </tr>
    <tr>

  </table>
  <p>&nbsp;</p>
</form>
</div>  

PHP code;

<?php
   session_start();

$server = "host.com";
$schema = "Seatter";
$uid = "user";
$pwd = "password";

    $password =$_GET["txt_Password"];
    $username =$_GET["txt_Username"];


    mysql_connect($server , $uid , $pwd) or die ("server not found");
    mysql_select_db($schema) or die ("Database not found");

    $sql = "SELECT *
    FROM Login_Details
    WHERE User_ID = '$username' AND  Password = '$password'";
$record = mysql_query($sql) ;

    if(mysql_num_rows($record) == 0){

         echo "Login Failure: ";
     //echo $username; //Username read from html page
     echo $password; //Password not read.
     //header("location: Login.html");  
    }

mysql_close();

?>