Occurence and its seasonal variation of ozone-enhanced layers in the troposphere over euatorial Pacific Ocean were investogated based on ozonesonde data obtained at three Southern Hemisphere ADditional OZonesondes (SHADOZ) sites, Watukosek, American Samoa and San Cristobal, for 6 years between 1998 and 2003. The O3-enhanced layers were frequently observed at the three sites, and their occurrence was about 50% on average. The formation processes of O3-enhanced layers were investigated by meteorological analyses including backward trajectories. Several O3-enhanced layers resulted from the transport of air masses affected by biomass burning. The contribution of this process was about 30% at San Cristobal during February to March period and August to September period, while it was relatively low, about 10%, at Watukosek and Samoa. A significant part of the O3-enhanced layers were attributed to the transport of midlatitude upper-troposphere and lower-stratosphere (UT/LS) air. Meteorological analyses indicated that these layers were originated from equatorward and downward transport of the midlatitude UT/LS air masses through a narrow region between high and low pressure systems around subtropical jet stream. This process accounts for about 50% of observed O3-enhanced layers at Watukosek, about 80% of those at Samoa, and about 30% of those at San Cristobal, indicating that it was important for O3 budget over the equatorial Pacific Ocean.