The primary objective of this study is to investigate the size-composition distribution of environmental tobacco smoke (ETS) in indoor environment. Particle size is one of the major factors which influence the rates and sites of particle deposition in the human respiratory tract. Study of the size-composition distribution of the environmental tobacco smoke would therefore lead to a better understanding of its health effects. Samples of airborne particles were obtained from three billiard parlors in Taipei by a micro-orifice uniform deposit impactor (MOUDI(superscript TM)). While the original MOUDI(superscript TM) has eight stages, only four stages and the after- filter were used in this study. The cut sizes of the four stages are 0.17, 0.56, 1.8, 10μm respectively. The sampling flowrate was 30L/min and the duration of sampling was in the range of 3~5 hrs, depending on the indoor particulate concentration. Solanesol was chosen as a tracer in this study, because it is stable and has a relatively fixed mass ratio in tobacco smoke particles. The solanesol extracted from the samples was derivatized and then analyzed by the GC-FID method. The derivatization was made to avoid the decomposition of solanesol at higher temperatures. The results show that the mass concentration of fine particles (smaller than 2.5μm in aerodynamic diameter) is in the range of 53~461μg/m^3, with an average of 189μg/m^3; the solanesol concentration is in the range of 0.259~9.91μg/m^3, with an average of 3.03μg/m^3 the mass fraction of solanesol in fine particles is in the range of 0.0037~0.0244, with an average of 0.0150; and 10~32% (average 22%) of ETS are in particles larger than 0.56μm in aerodynamic diameter. The estimated contribution of ETS to fine particles is in the range of 70~93%.