Abstract: In this paper, a hydrodynamic numerical model including bottom friction is developed to calculate the instabilities of longshore currents on two barred beaches with respective slopes of 1:40 and 1:100. This model enables to estimate wavelength, propagation velocity, growth rate and perturbation velocity field of the instabilities. Here, the effects of the bottom friction on the instabilities are analysed according to the beach slope. The results show that the propagation velocity of the instabilities ranges between 0.71—0.79. Furthermore, the instabilities develop faster on the 1:40 slope beach than on the 1:100 slope beach. It is also shown that the bottom friction has a great effect on the inhibition of the instabilities and that backshear is a key parameter in the instabilities development. Therefore sand bars can often be observed in this zone. Finally, the strongest energy of the instabilities generally occurs at the crest of bar and the milder the slope, the longer the wave length of the instabilities.