Abstract: It was expounded that the ratio of oil film thickness to the characteristic length of the texture h/L and the reduced Reynolds number r_e are two key factors for the availability of Reynolds equation in the hydrodynamic lubrication simulation of textured surface. The Reynolds equation can be used safely only when both of the two key factors, h/L and r_e, tend to be zero. And a schematic illustration of the applicable range of Reynolds equation can be given on the h/L-Re plane. Further, the effect of h/L and re on the error of Reynolds equation was numerically calculated for the two dimension rectangle groove, and the invalidation mechanisms of Reynolds equation was discussed under different conditions. Finally, the evaluation criteria of Reynolds equation was proposed that the error of Reynolds equation can be within 10% when the reduced Reynolds number re and the h/L were smaller than 0.20 and 0.015 respectively.