Aerosol samples of fine particles (PM2.5) were collected simultaneously from inside and outside 16 residential homes in Tainan City of southern Taiwan. The collected samples were analyzed for the carbon content and 16 elements including Be, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Sb, Hg and Pb. The concentrations of mean elemental carbon (EC) and organic carbon (OC) of the indoor and corresponding outdoor PM2.5 were approximately the same and within the range of 10–12 μg/m3. The indoor/outdoor (I/O) ratios of EC and OC varied at 0.35–3.86 and 0.27–2.44, respectively. Metals such as Ca, Fe, Pb, and Zn were most abundant among the elements examined, while elements such as Be, Co, Hg, Cd were found in the smallest quantities. Using Fe as the reference element, the enrichment factors (EFs) varied from less than unity to more than 60,000. According to the degree of enrichment, As, Se, Cd, Sb, Hg were found to be the most enriched in PM2.5. The results suggest that the anthropogenic combustion sources such as coal combustion, municipal waste incineration, vehicular emission as well as building construction/renovation are the major sources of indoor PM2.5 in Tainan City.