<asp:CheckBoxList ID="CheckBoxList1" runat="server">
<asp:ListItem Selected="True">FirstCheckBox</asp:ListItem>
<asp:ListItem>SecondCheckBox</asp:ListItem>
<asp:ListItem>ThirdCheckBox</asp:ListItem>
<asp:ListItem>FourthCheckBox</asp:ListItem>
</asp:CheckBoxList>
<br />
<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Reset" />
protected void Button1_Click(object sender, EventArgs e)
{
for (int i = 0; i < CheckBoxList1.Items.Count; i++)
{
CheckBoxList1.Items[i].Selected = false;
}
}