This paper presents the results of an investigation on the interannual variations of tropical cyclone (TC) size, defined as the average radius of 15 m s-1 surface winds, over the western North Pacific from 1999 to 2002. The wind data are from QuikSCAT. The average TC size is found to increase from 1999 to 2002, with the TC sizes in 1999 and 2000 being significantly smaller than those in both 2001 and 2002. Differences in the environmental flow patterns associated with the tracks and formation positions of the TCs among these years apparently explain such size changes. Since 1999 and 2000 were La Ni¿a years, TCs in these two years formed more westward with fewer recurvers so that they tended to be smaller. On the other hand, with 2002 being an El Ni¿o year, TCs formed further eastward with more recurvers, and thus tended to be larger.