A case study of indoor and outdoor fine particles (PM2.5) was undertaken for rural areas in northwestern China, and quantitative data was obtained on their chemical composition including carbon fractions, water soluble ions, and elements. OM (organic matter), sulfate, and geological material dominated PM2.5, followed by nitrate and ammonium, which accounted for 78–85% of the mass for indoor and outdoor environments. The variations of the carbon fractions indicated that four OC factions and EC1 were more abundant in winter than in summer. SO42− contributions were the highest of the ionic species for indoor and outdoor environments (about 40% of total ions in winter and 53% in summer), followed by NO3− (about 23% in winter and 14% in summer). The integrated results from the ratios of K+/OC, K+/EC, and as well as the EF (enrichment factor) values for K, Cl, S and Pb, indicate that the biofuel contributions were significant in the rural area. The indoor/outdoor ratios and correlations of the components were also investigated. The results for the indoor and outdoor PM2.5 sources showed that biomass burning in summer was the dominant primary source (31% for indoor and 44% for outdoor), and those for winter were coal combustion (21% for indoor and 29% for outdoor) and biomass burning (24% for indoor and 16% for outdoor). Due to the local patterns of energy consumption, the discussion presented in this work could give implications for future strategies to improve rural air quality.