Fixed header in grid

Add the css:
<style type="text/css">
        .FixedHeader {
            position: absolute;
            font-weight: bold;
            vertical-align:text-bottom;
        }     
    </style>

<asp:GridView ID="gvDistricts" runat="server" HeaderStyle-CssClass="FixedHeader" HeaderStyle-BackColor="YellowGreen"
                    AutoGenerateColumns="false" AlternatingRowStyle-BackColor="WhiteSmoke" OnRowDataBound="gvDistricts_RowDataBound">
                    <Columns>
                        <asp:TemplateField HeaderText="District ID" HeaderStyle-Width="100px" ItemStyle-Width="101px">
                            <ItemTemplate>
                                <asp:Label ID="lblDistID" runat="server" Text='<%#Eval("district_code")%>'></asp:Label>
                            </ItemTemplate>
                        </asp:TemplateField>
                    </Columns>
                    <Columns>
                        <asp:TemplateField HeaderText="District Name" HeaderStyle-Width="300px" ItemStyle-Width="301px">
                            <ItemTemplate>
                                <asp:Label ID="lblDistName" runat="server" Text='<%#Eval("district_name")%>'></asp:Label>
                            </ItemTemplate>
                        </asp:TemplateField>
                    </Columns>
                    <Columns>
                        <asp:TemplateField HeaderText="Description" HeaderStyle-Width="200px" ItemStyle-Width="200px">
                            <ItemTemplate>
                                <asp:Label ID="lblDistDesc" runat="server" Text='<%#Eval("Description")%>'></asp:Label>
                            </ItemTemplate>
                        </asp:TemplateField>
                    </Columns>
                </asp:GridView>