I would not do this in SummaryValueChanged. I would do it when you add the summary, as I did in the code I posted above. Otherwise you will be continually setting and resetting the property to the same value, which is inefficient.

thanks,  so on the event for SummaryValueChanged I want to set the display format to the format of that specific column,  do you know how I can get the column name or index? 

Right now I am setting it like this.

        private void ultraGrid1_SummaryValueChanged(object sender, SummaryValueChangedEventArgs e)
        {
            e.SummaryValue.SummarySettings.DisplayFormat = "{0:###,###,###,###,###.##}";
        }

When you create your summary using Summaries.Add, the Add method returns a SummarySettings object.

SummarySettings summarySettings = band.Summaries.Add(...);

summarySettings.DisplayFormat = "{0:#,##0.000}";

can you give me some code on how to set it?

 all I could find was 

grid.DisplayLayout.Override.SummaryValueAppearance  and I cant specify a column for this. 

I set the column with

grid.DisplayLayout.Bands[0].Columns[colName].Format = "###,###,###,###,###.##";