Video communications from personal computers to 3G mobile phones have still not been implemented in the existing instant message systems. This paper discusses the general network architecture for instant message systems and innovatively presents a gateway from IP core network to WCDMA 3G mobile network in Hong Kong and devises a sort of protocols based on reduced Session Initial Protocol. Moreover, the article designs and implements a new instant message system that puts audio and video communications between personal computers and 3G mobile phones into practice. The running results testify that communications qualities meet the requirements of multimedia session. The system is exhibited at the ITU TELECOM WORLD 2006 and gets much attention, furthermore, it is be reported by many newspapers.