To understand the distribution characteristic and bioaccumulation factors(BAF) of total mercury(THg) and methylmercury(MeHg) in phytoplankton at Guizhou Plateau reservoirs, southwest of China. The phytoplankton and water samples were investigated at five reservoirs in the winter of 2012. The results showed that diatom was the dominant algae at Guizhou Plateau reservoirs in winter and phytoplankton abundance had obvious difference at the five reservoirs. The maximum phytoplankton abundance was about 29.05×10⁴ cells/L at Lake Baihua dam, and the minimum only 0.49×10⁴ cells/L at Sanbanxi Reservoir. The concentrations of THg and MeHg in phytoplankton were 29.29-277.07 ng/g(dry) and 1.12-10.93 ng/g(dry), respectively. The maximum concentrations of THg and MeHg in phytoplankton was at Lake Baihua. The BAF of THg and MeHg in phytoplankton were 1.42×10⁴-9.78×10⁴ and 3.10×10⁴-4.43×10⁵, respectively. The maximum BAF in phytoplankton of THg was at Lake Baihua. The maximum BAF of MeHg in phytoplankton was at Lake Hongfeng. There was different effect on biological enrichment of mercury and methylmercury for different phytoplankton species composition. There were significant positive correlations between MeHg in phytoplankton and THg, MeHg, dissolved MeHg (DMeHg) in water. No significant correlation was found between MeHg in phytoplankton and the THg, DMeHg in water. MeHg was not only absorbed by phytoplankton. There were other factors influencing Me-Hg concentration in phytoplankton inevitably. There were significant negative correlations between MeHg in phytoplankton and environmental factors (SD, TN, NO₃^--N). The degree of eutrophication had effects on MeHg concentration in phytoplankton.