The combined effects of a tube pitch and the heat flux of a lower tube on saturated pool boiling heat transfer of tandem tubes were investigated experimentally. For the test, two smooth stainless steel tubes of 19 mm diameter and the water at atmospheric pressure were used. The pitch was varied from 28.5 to 114 mm and the heat flux of the lower tube was changed from 0 to 110 kW/m(2). The bundle effect was clearly observed when the heat flux of the lower tube was greater than that of the upper tube and the heat flux of the upper tube is less than 60 kW/m(2). The maximum bundle effect was 3.08 as the tube pitch was 95 mm when the heat fluxes of the upper and the lower tubes were 10 and 90 kW/m(2), respectively. In general, the bundle effect was decreased with increasing the tube pitch. The major heat transfer mechanisms are considered as the convective flow of bubbles, rising from the lower tube, and liquid agitation caused by the bubbles. As a way of quantifying the bundle effects a simple empirical correlation was suggested. (C) 2015 Elsevier Ltd. All rights reserved.