To initialize DataRow’s default values handle InitNewRow event of the required ASPxGridView control and initialize the default values by using e.NewValues dictionary in the following manner:
<dx:ASPxGridView ID="ASPxGridView1" ... OnInitNewRow="ASPxGridView1_InitNewRow">...</dx:ASPxGridView>
protected void ASPxGridView1_InitNewRow(object sender, DevExpress.Web.Data.ASPxDataInitNewRowEventArgs e) {
e.NewValues["DataColumnForDefaultValue"] = DateTime.Now;
}