Abstract: Generally the problem of multiphase stock replenishment was studied with two assumptions, i.e., take inventory continuously to examine the whole quantity ordered at any time and take inventory cyclically to examine the quantity just ordered discretely. Differing from the two assumptions, the problem is studied further taking the replenishing interval as a continuous random variable in cyclical replenishment. This assumption reflects the real life that supplier usually visits a retailer randomly in a definite interval and the retailer shall replenish his stocks in accordance to the policy to control the replenishment up to a constant level. It is therefore deemed that only part of the unmet demand is backordered and the rest will lose the opportunity for selling. As a result, assuming that all the goods are perishable and their deteriorating rate is constant, a conditional equation of optimal stock level is given to maximize the mathematical expectation value of profit. It provides a reference for how to control actually the replenishment level.