An improved, rapid method to measure free-base nicotine in mainstream cigarette smoke accurately by commercial headspace solid phase microextraction (HS-SPME) combined with GC/MS was established. The factors which play important roles in the determination of free-base nicotine, such as the solvent used for preparation of standards, SPME fiber, pre-equilibrium time before HS-SPME, extraction temperature and time, desorption time, were optimized carefully. Furthermore, an experimental water correction factor based on the water content of particulate phase was established to correct free-base nicotine in actual samples. The precision of the method was evaluated with smoke samples of reference cigarettes (Kentucky reference 3R4F) and the RSD obtained was 8.3%. The calculated limit of detection (LOD) and limit of quantification (LOQ) were 0.2 and 0.5 μg/cig, respectively.