Hello, Deasun,
In order to keep the new lines in the cells values, you can handle the CellFormatting event of the SpreadExportRenderer and enable the text wrapping:
public RadForm1()
{
InitializeComponent();
this.radGridView1.Columns.Add("Col");
this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
this.radGridView1.Rows.Add("First"+Environment.NewLine+"Second"+Environment.NewLine+"Third");
this.radGridView1.AutoSizeRows = true;
}
private void radButton1_Click(object sender, EventArgs e)
{
string fileName = @"..\..\" + DateTime.Now.ToLongTimeString().Replace(":", "_") + ".xlsx";
GridViewSpreadExport spreadExporter = new Telerik.WinControls.Export.GridViewSpreadExport(this.radGridView1)
{
ExportVisualSettings = true,
FileExportMode = Telerik.WinControls.Export.FileExportMode.NewSheetInExistingFile
};
SpreadExportRenderer exportRenderer = new Telerik.WinControls.Export.SpreadExportRenderer();
spreadExporter.CellFormatting+=spreadExporter_CellFormatting;
spreadExporter.RunExport(fileName, exportRenderer);
Process.Start(fileName);
}
private void spreadExporter_CellFormatting(object sender, Telerik.WinControls.Export.CellFormattingEventArgs e)
{
e.CellStyleInfo.TextWrap = true;
}

I hope this information helps. If you need any further assistance please don't hesitate to contact me.
Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik
Get
quickly onboarded and successful with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers.
Learn More.