This study aimed to examine the prevalence and the socio-demographic correlates of mental health of migrant workers in Shanghai China. A total of 475 migrant workers from four major districts in Shanghai were recruited through a survey design with a multistage cluster [corrected] sampling. Male and female migrant workers were identified as mentally healthy or unhealthy using the brief symptom inventory. Socio-demographic characteristics and migration stress were explored as correlates of the mental health of the migrant workers. A total of 73 migrant workers could be classified as mentally unhealthy (25% for men and 6% for women). Male migrant workers who were married (OR 6.16, 95% CI 1.83-20.70), manual laborers (OR 1.56, 95% CI 0.97-2.51), and experienced more stress in "financial and employment-related difficulties" (OR 2.75, 95% CI 1.47-5.14) and "interpersonal tensions and conflicts" (OR 4.18, 95% CI 1.55-11.25) were more likely to be mentally unhealthy, whereas the female migrant workers who experienced more stress in "interpersonal tensions and conflicts" (OR 6.52, 95% CI 0.83-51.14) were more likely to have poor mental health. The findings provide information for the prevention of mental illness among migrant workers in China. The implications and limitations are also discussed.