High-rise high-density housing in the mega cities in China such as Hong Kong and Shanghai is perceived as a compact and sustainable development model. However, the outdoor thermal environment can be deteriorated, as indicated by the increasingly strong urban heat island (UHI) effect, due to extensive use of impervious materials, enhanced anthropogenic heat and loss in vegetation cover. This paper investigates the summertime thermal effect of urban form and vegetation factors at two selected high-rise residential quarters in Shanghai. The results show that, during the measurement period, the site A which has abundant greenery cover and leaf density was on average 0.8 °C cooler during the day and 0.45 °C cooler at night respectively, than the site B which features less greenery cover and less trees. Case study and regression analysis indicate that the sky view factor (SVF), total site factor (TSF), tree view factor (TVF), green plot ratio (GPR) and wind velocity (WVD) are significantly related to day-time UHI intensity; while SVF, GPR, TSF, TVF, green cover ratio (GCR) and wind velocity (WVN) are significantly related to night-time UHI intensity. The yielded multiple regression models explain up to 64% of variability in UHI-day and 53% of variability in UHI-night, respectively. Numerical modelling and simulation using ENVI-met was also performed. The findings can inform decision makers, urban designers and architects at early design stage in improving outdoor thermal comfort and reducing building cooling energy consumption.