The year-to-year ozone variation over the subtropical western Pacific region is studied, especially the ozone lows in the 1996/1997, 1998/1999, and 2001/2002 winters, using the Earth Probe Total Ozone Mapping Spectrometer (EP_TOMS) ozone data from August 1996 to July 2002. Regression analyses show that dynamical signals, such as the quasi-biennial oscillation, play an important role in determining total ozone variation. A nudging chemical transport model (CTM) is used to simulate the year-to-year ozone variation and explain the mechanism for producing ozone lows in a three-dimensional distribution of ozone. The CTM was developed using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model and introducing a nudging process for temperature and horizontal wind velocity. The year-to-year ozone variation, especially the winter ozone low, is well simulated by the model excluding heterogeneous reaction processes between 45¿S and 45¿N latitude. Results show that the year-to-year ozone variation is mainly controlled by dynamical transport processes.