This paper addresses the analysis of the retrieval lead time of the storage/retrieval(S/R) vehicle within an aisle when items are picked up by the batch size in a warehouse system. Batch size retrieval is to combine several orders in a single tour of the S/R vehicle within an aisle. An advantage of batch size retrieval is that the length of a tour for a batch of orders is shorter than the sum of tour lengths of the individual orders. The plan batch size is the number of items considered in a retrieval plan for retrieval of items from storage cells of the warehouse. The retrieval batch size is the number of items included in a single tour. The items included the plan batch size are retrieved from storage cell by the retrieval batch size. In case that the plan batch size is divided and retrieved by the retrieval batch size, the probability that each storage location row of the rack will be the location of the multiples of retrieval batch size is analyzed. The average travel distance and the retrieval lead time are analyzed for the plan batch size and the retrieval batch size. A problem is analyzed and the mathematical model is formulated, and a numerical example is showed to explain the solution procedure.