Abstract: Objective To understand the adverse effect of occupational exposure to ammonia on workers' health condition. Methods The health status of 218 workers occupationally exposed to ammonia were compared with that of 1 741 workers without exposure to ammonia in a petroleum refining plant. Results The abnormality of both RBC counts and PLT counts were higher in the male ammonia-exposed workers compared with non-exposed workers (χ²=17.46,P < 0.01;χ²=6.56,P < 0.05,respectively). The Hb and RBC counts among male exposed workers were lower compared with non-exposed workers (t=-2.46,P < 0.05;t=-5.08,P < 0.01,respectively),while PLT counts were lower among male and female exposed workers compared with non-exposed workers (t=-2.46,P < 0.05;t=-5.08,P < 0.01,respectively). The female exposed workers had a higher level of ALT activity compared with non-exposed workers (t=2.42,P < 0.05). The Logistic regression showed that ammonia exposure was a risk factor for decreased RBC and Hb counts of male exposed workers (OR=2.36,95% CI:1.56-3.56;OR=2.37,95% CI:1.07-5.23, respectively),a risk for decrease PLT level of both male and female exposed workers (OR=8.12,95% CI:2.02-32.69) and a risk for increased ALT activity of female workers (OR=3.25,95% CI:1.04-10.21). Conclusion This result indicated that ammonia exposure at current level in this petroleum refining plant caused adverse health effects and the further control measures should be taken to protect workers' health.