In order to understand the characteristics and relationships between indoor and outdoor PM2.5 during the heating period of 2014 in Beijing, the investigation of PM2.5 and associated species including organic and elemental carbon (OC/EC), water soluble ions, metal elements and trace organic matter (OM) were undertaken at a residential apartment. The average PM2.5 concentration was 55.2 ± 47.3 µg m–3 for indoor and 100.4 ± 82.1 µg m–3 for outdoor, and the indoor PM2.5 was found to be mainly from outdoors. OM and (NH4)2SO4 were the dominated components of PM2.5, accounted for 71.5% in indoor PM2.5 and 52.4% in outdoor PM2.5, followed by fine soil and NH4NO3 (23.7% and 27.9%). The polycyclic aromatic hydrocarbons (PAHs) concentration was 187.3 ng m–3 and 387.0 ng m–3, and the phthalic acid esters (PAEs) concentration reached 1054.2 ng m–3 and 515.3 ng m–3, for indoor and outdoor, respectively. Hexachlorobenzene (HCB) only existed indoors (5.5 ng m–3). HCB and most PAEs in indoor PM2.5 were dominated by indoor sources whereas other species were greatly influenced by outdoor sources especially during the pollution period.