Online discussions have become important educational activities through which collaborative learning and knowledge construction can be facilitated. This study explores the use of two online discussion tools, online discussion board and VoiceThread, for supporting online course discussion activities among a group of graduate students and investigates the differences in the levels of social presence and social knowledge construction in the two discussion settings. Meanwhile, the participants' posting behaviors and interaction patterns were also examined. Results reveal that the participants contributed more words when using the audio and video features of VoiceThread. Additionally, the graduate students tended to create more contents and demonstrated more social presence behaviors when interacting with others in the VoiceThread discussion platform. However, the participants exhibited fewer social knowledge construction behaviors than they did in the text-based online discussion board.