CellStyle cs = wb.createCellStyle();
cs.setDataFormat((short) 4);
cell.setCellStyle(cs);
cell.setCellValue(value);
cell.setCellType(HSSFCell.CELL_TYPE_NUMERIC);
// HSSFDataFormat df = workbook.createDataFormat();
// numStyle.setDataFormat(df.getFormat("#,##0.00"));
http://blog.youkuaiyun.com/zxcvbnmluton/archive/2010/12/10/6066916.aspx