The effect of the wall to the smectic layer ordering is studies at thin liquid crystalline system by generalizing the McMillan theory on smectics. Every order of smectic layers is proved not to vanish, and a crossover from a long range order to a surface sustained one is shown to occur near a bulk transition temperature which is explained as the critical behaviour even at such thin system. The dependence of transition temperature between smectic phase and nematic one at the parallel aligned case is obtained, where the transition temperature is determined from the global behaviour of the order averaged over the whole system.