Differential chemical etch method (DCEM) was employed to study the effect on the surface/subsurface damage depth of K9 glass lapped by different particle sizes of fixed diamond abrasives. The advantage of DCEM is that both the lapped and substrate samples are placed in the chemical etch at the same time to decrease or eliminate the effect of etching condition variation. K9 glasses are firstly fixed-abrasive lapped with 40, 28, 14 and 10μm diamond abrasives, respectively. Surface damages of K9 glass after FAL are measured by Microscopy. The results show that the corresponding subsurface damage (SSD) depths of K9 glass are 4.5, 3.4, 2.8 and 1.6μm when the fixed diamond abrasive particle size are 40, 28, 14 and 10μm.With the decreasing of diamond particle size, the SSD depth decreases obviously and surface quality of K9 glass is improved significantly.