How users can use Grid effectively is a key issue in the research on interface and utilization mode of Grid.In this paper

the definitions of 3A(Any time

Any place and Any device) utilization mode was presented

and a formal model of user and service grid system (USG) was built using ASM.And based on USG model and these formal definitions

a theorem on 3A utilization mode was also be proposed

and the theorem was proved that if USG can satisfy the user validity

session connectivity and service continuity

then users can use service grid at any time

any place

and on any device.