Graphene has recently become one of the best candidates for ultrasensitive gas detector, due to its huge specific surface area and good conductivity of heat and electricity. In this study, a density functional theory (DFT) calculation is proposed to explore NO adsorption on graphene and Cr doped graphene. Compared with electronic structures of the two systems, it is found that the Cr substitution significantly enhances the adsorption behavior of NO molecules (adsorption energy being increased to -1.58 eV), while more electrons transfer from the substrate to the adsorbate (0.143 e). Therefore, this Cr doped graphene is expected to be an excellent candidate for sensing NO gas.