Due to urbanization, the urban heat island (UHI) effect has become a modern endemic threat to human health and energy consumption, especially in compact urban areas with high population density. Pocket parks, as the major component of urban green space in high-rise high-density built area of Hong Kong, play important roles in shaping the city. By conducting in situ climate measurements and morphological analysis of 12 sites in Hong Kong, this study investigates the impact of pocket parks on UHI intensity, and the relationships between five urban planning indicators and UHI. Results show that pocket parks and their surrounding areas in Hong Kong have a moderate UHI intensity (0.1 °C) during the daytime but a very high nighttime UHI intensity (2.39 °C). Ten parks are cooler than their surrounding urban streets for both daytime and nighttime, which means that pocket parks in Hong Kong can help to alleviate UHI intensity at the micro scale. Floor Area Ratio (FAR), building density and Tree Cover Ratio (TCR) are found to be significantly and negatively related with daytime UHI. It indicates that higher FAR, building density and TCR in high-rise high-density urban environment may help to reduce daytime UHI intensity without increasing the early nighttime intensity. Planting trees is an effective measure to reduce the UHI intensity inside the parks for high-rise high-density urban environment. But a threshold for TCR (around 42%) shall be achieved to maximize its cooling effect.