The APEX project (Asian Atmospheric Particle Environmental Change Studies) has been initiated by one of JST (Japan Science Corporation)/CREST (A Core Research Project for Evolutional Science and Technology) projects for the period of 1999-2004, in order to understand and model the aerosol-cloud interaction phenomenon, known as the aerosol indirect effect on climate, in the Asian region. As reviewed by IPCC (1996, 2001), there is a large uncertainty in the evaluation of the indirect effect of man-made aerosols ranging from 0 to -2 W/m2 in terms of the global mean radiative forcing. The main emphasis of the APEX project is to understand and model the indirect climate effect of aerosols by combined efforts of surface and aircraft measurements, active lidar/radar sensing, satellite-remote sensing, and numerical modeling. Intensive field observations, named as APEX-E1 and -E2 experiments, were performed in Dec. 2000 and April 2001, respectively, in order to integrate our efforts to study the detailed condition in the East China Sea area. The latter experiment was in callaboration with the ACE-Asia experiment. The efforts now start producing quantitative estimates of geophysical parameters of aerosols and clouds, along with the radiation budget in the Asian region. The summary of our current knowledge suggests that the TOA radiative forcing of the direct and indirect effects of man-made aerosols are respectively -0.2 W/m2 and -1 W/m2 as the global mean, indicating the indirect effect may be the major cooling effect competing with the global warming due to greenhouse gas increase.