Abstract The fuel temperature coefficient of reactivity is an important parameter in the evaluation of transients in light water reactors. The fuel temperature coefficient ( α f = 1 k 2 · ∂k ∂T f of an infinite lattice, often used in pin-cell calculations, is not a correct measure for the fuel temperature effect in such a lattice. We present a somewhat different definition of the fuel temperature coefficient of reactivity ( α ∗ f = 1 k · ∂k ∂T f ). This definition gives a more correct measure for the fuel temperature effect. Some calculations on lattices which are characteristic for a PWR show its accuracy. The newly defined fuel temperature coefficient is much less dependent on the fuel enrichment and the 10B-concentration. The fuel temperature co-efficient is analysed by calculating the different components constituting this coefficient.