In this paper, the program UV calc, utilizing the multiple point source summation approximation (MPSS) for a linear lamp, was used to compute the ultraviolet (UV) dose distribution and average intensity inside a UV disinfection reactor. Simultaneously, Bacillus subtilis was used for bioassays. The results demonstrated that the doses calculated from theoretical computation were significantly higher than those from the bioassay and the lowest dose area was close to the wall of reactor between the central axis of the two UV lamps. Also, the discrepancies between the theoretical computation and bioassay were correlated with flow rate and UV transmittance. The data from computation were in close agreement with those from the bioassay under conditions of high flow rate and low transmittance. compared to theoretical computation, bioassay is the credible method to confirm the fluence response curve of UV reactor.