A boat for semiconductor wafers includes first, second and third support rods arranged between and connected to top and bottom plates. The first, second and third support rods include a plurality of first racks, a plurality of second racks, and a plurality of third racks, respectively, such that the racks of each rods are vertically arrayed with gaps therebetween. The first, second and third racks serving to define a plurality of horizontal wafer supporting levels. Each wafer supporting level is defined by only combination of the first, second and third racks of the corresponding height. In each wafer supporting level, the first and second racks are arranged substantially in symmetry with respect to an axis passing through the center of a wafer transfer port, and the third rack is arranged deviant from the axis by a certain distance corresponding to 5% to 48% of the diameter of the wafer.