Ground-based PM_2.5 observations were compared with the space-borne observed aerosol optical depth (AOD). Decadal-scale observations at 5 sites in Japan and the observation by the U.S. embassy at Beijing in China were utilized. Measurements by MODIS onboard Terra and Aqua have been detecting AOD from space since February 2000 and July 2002, respectively. The monthly averaged PM_2.5 and AOD at each site corresponded with correlation coefficients around 0.7 both in the morning and afternoon except for Osaka and Beijing. The coverage threshold of monthly data in MODIS dataset was considered as an important factor to evaluate the correspondence of AOD with ground-based PM_2.5, especially at Beijing. When the threshold value of MODIS monthly coverage was set at 40%, the averaged correlation coefficient at 6 sites in Japan and China showed correlation coefficient of 0.63 with a statistically significance level. PM_2.5 and AOD also had a spatial correlation. As an example of application, on the basis of sound relationships between PM_2.5 and AOD, episodic events of high PM_2.5 concentrations in Beijing occurred during January 2013 were assessed, and the results indicated that a dramatic increase of PM_2.5 around Japan were not found during that episode.