Distributed time service is a basic service in distributed object computing environment based on CORBA. In this paper, we present a clock synchronization method based on clock groups. We dividing machines in a system into groups, nodes in a group are in the same LAN. There is a master clock and several slave clocks in a group, who carry out intra-group synchronization. Master clocks of all groups together accomplish inter-group synchronization. Such dividing-to-group synchronization method make the system scalable and flexible. At the same time, it reduces machines spanning LANs to attend synchronization, thus synchronization overhead is reduced. We implemented clock synchronization based on clock groups in our experimental distributed object environment.