Fighting tree encroachment using fire promotes C4 perennial grasses but likely affects their demography according to the fire date during the dry season. We analyzed the impact of four fire treatments (early, middle, late and no fire) on the demography of the four dominant perennial grasses (Andropogon canaliculatus, Andropogon schirensis, Hyparrhenia diplandra and Loudetia simplex) in a Guinean savanna of West Africa (Lamto, Côte d'Ivoire). We carried out a yearly demographic monitoring of each grass individual during five years (2015\textendash 2019) on three plots by treatment and parametrized a size-classified matrix model with five circumference classes. The results showed that A. schirensis, H. diplandra and L. simplex declined (λ < 1.0) under late fire and will disappear after 10 years, as did L. simplex under the middle fire. Stasis influenced the most λ values and stable class distribution was nearly achieved in all species under all treatments. The size of L. simplex should increase under early and late fires. Our results suggest that late fire is the most detrimental fire regime for grasses in this Guinean area, contrary to early and middle fires, which could be recommended to savanna managers.