Early warning of the potential of mutagens or carcinogens caused by benzene exposure that might occur ingasoline station workers can be achieved by examining 2 major biomarkers: sister chromatid exchange (SCE)and trans, trans-muconic acid (t,t-MA), a urinary metabolite of benzene. The main objective of this study was toassess benzene exposure and monitor the genotoxic effect of gasoline station workers in Bangkok, Thailand. Bloodand urine samples were collected from 33 gasoline station workers, working in Pathumwan district area, centralBangkok, Thailand, for SCE and t,t-MA analysis, from April to June 2009. Control samples were collected from30 office workers and students in the same area at the same period. Our results indicated significantly higherfrequencies of SCE in gasoline exposed workers were than in controls (p<0.01), independent of gender. Urinaryt,t-MA and t,t-MA/creatinine levels of gasoline exposed workers were also significantly higher than the controlgroups (p<0.05) were significantly higher in women than men workers (p<0.01). Calculated chromosomal damagerelative risk (RR) of gasoline station workers was 3.00 (95% CI = 1.81 - 4.98, p<0.001) compared to controls. Thegasoline exposed workers had potentially higher risk of chromosomal damage and cancer development becauseof direct contact to benzene.