I already give this solution on many sites like stack overflow & Obout website . So I am explaining again on my blog.
This is a limitation of the standard Unit class: http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.unit.pixel.aspx "The Unit class can represent values only between -32768 and 32767." So you need to make sure that the sum of all the columns' widths is smaller than 32767. The default width of a Column is 175px, so if you auto generate the columns you will be able to use up to 187 columns. If the columns are auto generated, you can reduce the width of the columns using the ColumnsCreated event: http://www.obout.com/grid/aspnet_events_columns.aspx If the columns are manually defined using the Columns collection simply set a smaller width for some of them.