Abstract: The sources of organic carbon and nitrogen and the seasonal changes of their suspended particulate matter from inflow river of Erhai Lake were studied by stable isotope techniques. The relationships of the two contents with natural and anthropogenic activities were also discussed. The results showed that ratios of the carbon and nitrogen isotopes of suspended particulate matters were -28.6‰ to -14.4‰ and 3.4‰ to 7.9‰ in spring,-25.1‰ to -16.9‰ and 5.4‰ to 10.6‰ in summer,-30.0‰ to -10.7‰ and 6.2‰ to 8.7‰ in fall, and -20.9‰ to -11.0‰ and -0.5‰ to 8.8‰ in winter, respectively. The seasonal changes of their isotopic compositions were observed in the study area. The C₃plants and aquatic plants were mainly the mixed source of the organic matter in spring, and the contribution of C₃plants source gradually increased; in summer it mainly came from land-based sources of C_3plants; in fall it was still C_3plants and aquatic plants mixed source, but the contribution of aquatic source increased; when winter, C_4plants mixed into C_3and aquatic plants source. The nitrogen source in the spring derived from soil erosion and aquatic plant residue, but the contribution of soil erosion was higher; it mainly came from soil erosion in summer; mixed soil erosion, fertilizer and the death of algae in fall; soil erosion, fertilizer and plant residues were the source and the contribution of nitrogen from fertilizer increased in winter.