You need to add 'checked' htmlAttribute in RadioButtonFor, if the radiobutton's value matches with Model.Gender value.
@{
foreach (var item in Model.GenderList)
{
<div class="btn-group" role="group">
<label class="btn btn-default">
@Html.RadioButtonFor(m => m.Gender, item.Key, (int)Model.Gender==item.Key ? new { @checked = "checked" } : null)
@item.Value
</label>
</div>
}
}
For complete code see below link: To render bootstrap radio button group with default checked. stackoverflow answer link