Objective To understand the characteristics of indoor PM2.5 pollution in residential houses in Guangzhou. Methods Indoor PM2.5 samples were collected in nine residential sites using mini-volume sampler in Guangzhou in summer (June to July, 2003). Mass concentration of PM2.5 and the organic carbon (OC) and elemental carbon (EC) levels in PM2.5 were measured. Results The average concentration of PM2.5 was (47.4?17.7) mg /m3, a relatively high concentration compared to the previous studies in other cities. The OC and EC concentrations in PM2.5 were (12.5?8.7) and (4.4?2.3) mg /m3 respectively. OC and EC counted for 24.6%?5.6% and 9.9%?4.8% in indoor PM2.5. However, it was estimated that OC mainly came from indoor sources and EC related well with outdoor sources, which conformed to the source investigation. Conclusion Cooking, smoking and outdoor sources are the prominent contributors to the indoor PM2.5 pollution. Housing conditions such as decoration and ventilation may affect the indoor PM2.5 level.