In this paper we present numerical evaluations of influence of accumulation effects on the heating of silicon surface by femtosecond laser pulses at low repetition rate in the regimes of formation of laser-induced periodic surface structures. Numerical evaluations were made using combined method, where photoexcitation process of semiconductor is described by diffusion equation of nonequilibrium carriers, electron subsystem and lattice subsystem heat is calculated using two-temperature model and the cooling of semiconductor between the pulses is described by analytical solution of the heat equation. The change of silicon absorptivity during the pulse, the change of the absorptivity between the pulses due to the formation of the periodic structures and a contribution of recombination processes is taken into the account. A comparison of the results with experimental data on the evolution of the microstructures during femtosecond laser irradiation of silicon is made. (C) 2015 Elsevier B.V. All rights reserved.