Urban green space as a semi-natural element in cities is usually used by city planners to alleviate pollution, improve landscape quality of human settlements and meet the psychological needs of urban residents close to nature. It is significant to investigate and assess urban residents' cognition in services of green space for a better green space system plan and management. The social services of green spaces and cognition of residents have been investigated with questionnaire method in eight different communities,including urban zone and suburb communities, old and new communities in Nanjing city. The questionnaire is constituted by three parts: the interviewees' background, green space condition and perception and awareness of residents to green space function. The data from questionnaires have been recorded and statistically analyzed through SPSS. The K Independent Samples and K Related Samples of nonparametric test and One-Way ANOVA of compare means have been adopted. The results show that the awareness level of green space services increase in the order of rising house price, advancing neighborhood image, strengthening communication, landscape, recreation place, improving environment; the perception level of green space services increase in the order of improving environment, rising house price, enhancing community identity, landscape and recreation place. The two indicators are broadly consistent except for cognition of ecological environmental service of green space. The objective perception level of ecological environmental service is less than the subjective awareness level. The interviewees' background and green space condition are factors influencing cognition level. The perception and awareness levels have been statistically verified to vary by residents' characteristics in age and educational level, and the green volume and biodiversity of plant. Finally, we suggest that urban green space planning should pay more attention to residents' cognition of green space service, especially for the function of place and landscape. Green volume and plant species diversity should be improved so that residents cognize the ecological environmental function of urban greenspace.