Instant Messaging (IM) has become and will continue to be an important and popular tool in Macao. That's why it is worth attention. It can be seen that IM has brought some impact on other communication tools, media and even on people's life in one way or another. This research adopted the notions of uses and gratifications theory, rich media theory, media displacement effect and the concepts of weighted and calculated needs for new media in order to learn more about IM. Thus, a survey was conducted in University of Macau to single out the gratifications that university students gained by MSN and how they use this tool. The author analyzed the data from 484 respondents. Results showed that students use MSN for "Entertainment", "Excitement", "Social Interaction", "Convenience" and it may in some way consider using MSN a "Habit". Another purpose of this research is to find out the relationship between IM (specific in MSN) and email. Results suggested that their relationship is complementary, which means that IM (MSN) and email cannot totally displace or replace one another. Students would weight and calculate their needs and choose the better tool in order to satisfy their different needs in various situations.