The reuse of wastewater for agricultural irrigation may cause human health risk as a result of exposure to pathogens. This study conducted the quantitative microbial risk assessment in paddy field irrigated with treated wastewater. Six treatments were used to irrigate the paddy field from Year 2003 to Year 2005: biofilter-effluent, UV-disinfected water (6, 16, 40, 68 $mW s cm^{-2}$), pond-treated water, wetland-treated water, conventional irrigation water and tap water. Total coliforms, fecal coliforms and E. coli were monitored during rice growing period. Beta - Poisson model was employed to calculate the microbial risk of pathogens ingestion that may occur to farmers and neighbor children. Uncertainty of risk was estimated using Monte Carlo simulation. In this study, the microbial risk was higher during initial cultivation (end of May$sim$June), and it decreased with time. Biofilter effluent (secondary effluent) irrigation showed higher risk values than others (>$10^{-4}$) and irrigation with UV-disinfected water has the lowest risk range ($10^{-6}{sim}10^{-5}$). The risk value estimated in 2005 was lower than risk value in 2003 and 2004, it is likely due to clean tap water irrigation in initial transplanting stage. It is suggested that irrigation with UV-disinfected water and pond-treated water would reduce the microbial risk associated with wastewater irrigation in paddy field. In addition, the first irrigation water quality significantly affected the subsequent microbial risk.