how to display bold?

WorksheetRow[] row = new WorksheetRow[Rep.Tables[0].Rows.Count];
                WorksheetRow[] rowz = new WorksheetRow[3000];

                for (int i = 0; i < Rep.Tables[0].Rows.Count; i++)
                {
                    string str = "", s = "Monthly Modeling";
                    str=dt.Rows[i][0].ToString();
                    row[i] = sheet.Table.Rows.Add();
                    if (str != emp)
                    {
                        rowz[i] = sheet.Table.Rows.Add();
                        rowz[i].Cells.Add(s + str );//This will be displayed in excel,how to make it Bold???
                        row[i] = sheet.Table.Rows.Add();
                    }