Consider the secnod order linear differential system with periodic coefficients
π=A(t)x (*)
and the stability problem of its trivial solution.In this paper,there are several conditions which are given including a necessary and sufficient condition for asymptotic stability,a necessary and sufficient condition for some kind of unstability, and a sufficient condition for stability.These conditions are expressed directly by coefficients A(t) instead of characteristic values of the system (*). Therefore,a feasible method of judging stability of the system(*) is offered.