Abstract: In today's realtime communication systems,direct video communications between personal computers(PCs) and the third generation(3G)mobile phones are still not available.In this paper,we use simple 3G modem cards to design and implement a gateway from IP core network to Wideband Code Division Multiple Access(WCDMA)3G mobile network in Hong Kong.A simplified Session Initial Protocol(SIP)is designed to implement realtime video and voice communications between PCs and 3G mobile phones.By a series of experiments,we testify the efficiency and robustness of the proposed realtime multimedia system.