The effect of the wall to the smectic layer ordering is studies at thin liq
uid crystalline system by generalizing the McMillan theory on smectics. Eve
ry 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 t
ransition temperature. The dependence of transition temperature between sme
ctic phase and nematic one at the parallel aligned case is obtained, where
the transition temperature is determined from the global behaviour of the o
rder averaged over the whole system.