  Based on local gauge invariance, four different kinds of fundamental
interactions in Nature are unified in a theory which has $SU(3)_c \otimes
SU(2)_L \otimes U(1) \otimes_s Gravitational Gauge Group$ gauge symmetry. In
this approach, gravitational field, like electromagnetic field, intermediate
gauge field and gluon field, is represented by gauge potential. Four kinds of
fundamental interactions are formulated in the similar manner, and therefore
can be unified in a direct or semi-direct product group. The model discussed in
this paper can be regarded as extension of the standard model to gravitational
interactions. The model discussed in this paper is a renormalizable quantum
model, so it can be used to study quantum effects of gravitational
interactions.
