Secondary organic aerosol (SOA) is an important constituent of airborne fine particles. PM_2.5 (particles with aerodynamic diameters ≤ 2.5 μm) samples were collected at a mountainous site in Hong Kong in autumn of 2010, and analyzed for SOA tracers. Results indicated that the concentrations of isoprene SOA tracers (54.7 ± 22.7 ng/m³) and aromatics SOA tracers (2.1 ± 1.6 ng/m³) were on relatively high levels in Hong Kong. Secondary organic carbon (SOC) derived from isoprene, monoterpenes, sesquiterpenes and aromatics was estimated with the SOA tracer based approach, which constituted 0.35 ± 0.15 μg/m³ (40.6 ± 5.7%), 0.20 ± 0.03 μg/m³ (30.4 ± 5.5%), 0.05 ± 0.02 μg/m³ (5.6 ± 1.7%) and 0.26 ± 0.20 μg/m³ (21.3 ± 8.2%) of the total estimated SOC. Biogenic SOC (0.60 ± 0.18 μg/m³) dominated over anthropogenic SOC (0.26 ± 0.20 μg/m³) at this site. In addition to the total estimated SOC (17.8 ± 4.6% of organic carbon (OC) in PM_2.5), primary organic carbon (POC) emitted from biomass burning also accounted for a considerable proportion of OC (11.6 ± 3.2%). Insight into the OC origins found that regional transport significantly (p < 0.05) elevated SOC from 0.37 ± 0.17 to 1.04 ± 0.39 μg/m³. Besides, SOC load could also increase significantly if there was influence from local ship emission. Biomass burning related POC in regional air masses (0.81 ± 0.24 μg/m³) was also higher (p < 0.05) than that in samples affected by local air (0.29 ± 0.35 μg/m³). Evidences indicated that SOA formation was closely related to new particle formation and the growth of nucleation mode particles, while biomass burning was responsible for some particle burst events in Hong Kong. This is the first SOA study in afforested areas of Hong Kong.