Large-scale monsoon gyres and the involved tropical cyclone formation over thewestern North Pacific have been documented in previous studies.This study is aimedto understand how monsoon gyres affect tropical cyclone formation.As the first part of this study,an observational study is conducted on monsoon gyres during the period2000-2010,with a focus on their structures and the associated tropical cycloneformation.A total of 36 monsoon gyres are identified in May-October during 2000-2010,among which 31 monsoon gyres are accompanied with the formation of 43 tropicalcyclones,accounting for 20.3％ of the total tropical cyclone formation.Monsoongyres generally are found on the poleward side of the climatological monsoon troughwith a peak occurrence in August-October.Extending about 1000 km outward from the center in at lower levels,the cyclonic circulation of the composited monsoon gyreshrinks with height and is replaced with negative relative vorticity above 200 hPa.The maximum winds of the composited monsoon gyre appear 500-800 km away from the gyre center with a magnitude of 8-13 m s-1 at 850 hPa.In agreement with previousstudies,the composited monsoon gyre shows enhanced southwesterly flows andconvection on the south-southeast side due to Rossby wave energy dispersion.Mostof tropical cyclones associated with monsoon gyres are found to form near the centersof monsoon gyres and the northeast end of the enhanced southwesterly flows,accompanying with weak vertical wind shear.