i am create html page in that i have to two div inside other
My issue is one div is for container and other for header

i made the margin 0px in both the div
but it showing the gap between the main window and first div

use this code
body {
   margin:0px auto;
   padding:0px;
}

All browsers have a set of default styling that's applied to HTML elements. What you should consider is applying a CSS "reset" so you can override the defaults. Its common in a reset file to apply margin and padding to 0 on many elements. This takes care of the mysterious space(s) you see on a variety of elements.

css code 
.mainpage
    {

       margin:0px;
       padding: 0px;
         font-family: "Times New Roman", Georgia, Serif;

    }
    .container
    { 
     font-size:14px;
      margin: 0px;
      border:1px solid black;
    }

    html code
    <body class="Main_page">
    <div class="container" >