Many studies reported that green roofs can notably alleviate roof-level surface and air temperatures, cutting building cooling loads and energy consumption. However, few studies have explored how large-scale roof greening affects the three-dimensional thermal environment of the urban canopy layer and how green-roof thermal performance correlates with urban geometry. This research investigated the block-scale green-roof thermal performance and its relationships with urban geometry factors by applying the three-dimensional microclimate model ENVI-met 4.2 in Nanjing City. Eight study plots with distinguished urban forms in conjunction with two orientations and three roof greening options generated a total of 48 scenarios for ENVI-met modeling and analysis. Results suggested that block-scale roof greening not only cooled the roof-level ambient atmosphere, but could ameliorate the ground-level microclimate. At 14:00 during the day, the air temperature reduction at 1.5 m height above the roof surface could reach 0.64℃ with an average reduction of 0.44℃. The maximum and average air temperature reductions at 1.5 m height above the ground were 0.55℃ and 0.25℃, respectively. Building height, height-to-width ratio, and floor area ratio had significant negative correlations with both the roof and ground level thermal effects. Building density was positively related with roof-level thermal effects, but had no significant correlation with ground-level thermal effects. Overall, compact low-rise and open low-rise building areas exhibited much higher cooling effects than compact and open high-rise building areas. Study sites with S-N orientation had better thermal performance than SW-NE orientated sites. These findings can deepen our understanding of building-vegetation-climate interactions in urban areas and provide a reference for roof greening practice in humid subtropical cities.