The overall heat transfer performance of a tube bank fin heat exchanger is very important for engineering applications. Developing a fin pattern with good heat transfer performance for tube bank fin heat exchanger needs more our intensive effort. There are two methods to obtain the heat transfer performances of a fin pattern, i.e., one is experimental method, and the other is numerical method. If numerical method is used, the thermal boundary condition on the fin surfaces is necessary. Generally, there are two ways to treat the thermal boundary, i.e., one is to treat fin surface with uniform temperature, and the other is to use a conjugate numerical method. The former is very easy to be applied in numerical method, but the latter needs more numerical effort. This paper reports the condition under which whether a conjugate numerical method or a numerical method just specifying uniform temperature thermal boundary condition should be used. It is found that such condition is the fin efficiency. When the fin efficiency is less than 0.8, a conjugate numerical method must be used. Otherwise, the numerical results obtained by applying an uniform temperature thermal boundary condition on the fin surfaces has only slightly differences with the results obtained by a conjugate numerical method. The reported results will provide a criterion for the researchers to choose a suitable numerical method in finding a fin pattern more efficiently and reliably. (C) 2012 Elsevier Ltd. All rights reserved.