An analytical study was performed to investigate the utility and accuracy of the lumped parameter approach to heat transfer analysis in a nuclear reactor subassembly. It is shown that the lumped parameter approach is a useful mathematical method if the appropriate heat transfer length scales associated with diffusion heat transfer are employed. Important results of this study supporting the utility of the lumped parameter approach indicate that in the parameter range of interest the length scales are geometry-dependent only. They are shown to be independent of radial power distribution across the subassembly, and asymptotic values which are equal to length scales corresponding to the thermally fully developed condition may be used in the thermally developing entry region which encompasses the entire axial length of typical nuclear reactor subassemblies. Two characteristic length scales are presented and may be easily applied in lumped parameter steady-state heat transfer calculations in spacer-free triangular arrays of nuclear fuel elements within the range of parameters of interest for nuclear reactor applications. These length scales apply to fuel rods adjacent and nonadjacent to subassembly walls, respectively.