In order to investigate the occurrence status of mercury in Xiaolangdi Reservoir, water, surface sediments, pore water and fish samples were collected and analyzed for the contents of total mercury (THg) and methylmercury (MeHg). Cold vapor atomic fluorescence spectrometry and aqueous phase ethylation derivatization-gas chromatography-atomic fluorescence spectrometry methods were used for the analysis of THg and MeHg, respectively. Bioaccumulation of THg in fish of Xiaolangdi Reservoir was then discussed. It was found that THg contents in water were 0.71-1.42 ng x L(-1) and 0.90-2.49 ng x L(-1) during dry and wet seasons, respectively, while the MeHg content in water was below the method detection limit. The THg content in water during both seasons could meet the requirement of Class I criterion of national surface water environmental quality standard (GB 3838-2002). The THg contents in sediments were 95.66-172.52 ng x g(-1) and 51.74-90.42 ng x g(-1), while the MeHg contents in the corresponding sediment samples were 0.18-0.39 ng x g(-1) and 0.09-0.26 ng x g(-1) for dry and wet seasons, respectively. The ratios of MeHg/THg in sediment were relatively small in comparison to those in other aqueous systems. This might be attributed to the high dissolved oxygen content of bottom water and low organic carbon content of the sediments. The THg concentrations in pore water of sediments were 5.46-41.04 ng x L(-1) and 4.27-9.49 ng x L(-1), and the MeHg concentrations were 0.07-1.01 ng x L(-1) and 0.09-0.99 ng x L(-1), respectively. The THg concentration was significantly higher than that of the overlying water, indicating the diffusion of Hg from the sediment to the water. The THg concentration in fish muscle tissues varied from 43.47 to 304.98 ng x g(-1), while the MeHg concentration varied from 10.77 to 265.23 ng x g(-1). The MeHg content in fish muscle tissues could meet the requirement of national food safety standards( GB 2762-2012) (Non carnivorous fish 500 ng x g(-1), Carnivorous fish 1 000 ng x g(-1)). The bioaccumulation factors of THg were 1.3 x 10(5) for bighead carp, 9.3 x 10(4) for barracuda, 4.7 x 10(4) for crucian carp, 5.0 x 10(4) for sharpbelly, 1.7 x 10(5) for yellow catfish, and 3.9 x 10(4) for arch fish, respectively.