Although a few studies have identified positive association between green space and reduced mortality rate, the effect modification of green space for the impact of air pollution on health outcomes is under studied. We quantified whether green space modifies associations between short-term exposure to particulate matter (PM10, PM2.5) and hospitalization across 364 urban U.S. counties for 2000-2013. Green space was measured by normalized difference vegetation index (NDVI). Daily number of hospital admissions for cardiovascular or respiratory diseases from Medicare enrollees (≥65yrs) and air quality monitoring data for each county were used to assess risks, as percent change in hospitalization related to 10μg/m3 increase in particulate matter. We computed an absolute change in county-specific relative risks explained by difference in county-level NDVI. The study results found that the association between air pollution and health was less in areas with more green space. We estimated that an interquartile range increase in NDVI corresponds to a 1.68% (95% CI: 0.43, 2.91) decrease in the association between PM10 and cardiovascular hospitalization and 10.40% (95% CI: 7.34, 13.34) decrease in the PM10-hospitalization association of acute myocardial infarction. For hospitalization associated with PM2.5, a 0.18% (95% CI: -0.39, 0.73) absolute decrease in relative risk was found for cardiovascular hospitalizations. In results stratified by age, younger age groups (65-74, 75-84yrs) had larger reductions for the PM10-hospitalization association with increase in NDVI than older populations (≥85yrs) but not for the PM2.5-hospitalization association. These findings add evidence for health benefits of green space in diminishing the health impacts of particulate matters on hospitalizations for older populations in the U.S.