Following is the code
SPList splist = web.Lists["MyList"];
var query = from SPListItem oItem in splist.Items.OfType<SPListItem>()
where !Convert.ToString(oItem["Status"]).Equals("Completed")
select oItem;
List<SPListItem> value = query.ToList<SPListItem>();
upto here working perfectly..
Now store value into viewstate getting following exception
ViewState.Add("Key", value);
Exception
System.Runtime.Serialization.SerializationException: Type 'Microsoft.SharePoint.SPListItem' in Assembly 'Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' is not marked as serializable.
Thanks