A method for upscaling relative permeability of a phase of a fluid in a reservoir model, the method being implemented by a computer, and comprising the steps of: - defining a reservoir model comprising a volume of dimensions D H, D V along respectively two distinct directions H,V, - receiving statistical data relative to the volume, comprising: ∘ relative proportions of at least two rock types, wherein each rock type corresponds to a permeability value and respective curves of relative permeability with water saturation of two phases of a fluid within the rock type, one of the phases being water, and ∘ a variogram of absolute permeability defined by correlation lengths L H, L V, along the two directions H,V, and, - computing, equivalent relative permeability values of a phase of the fluid within the volume, comprising: ∘ at least an equivalent relative permeability value (Kr eq,H) according to the first direction H, and ∘ at least an equivalent relative permeability value (Kr eq,V) according to the second direction V, characterized in that each equivalent relative permeability value of a phase of the fluid according to a direction d chosen among H,V is computed based on relative permeability values of the phase of the fluid within each rock type, and on a coefficient depending on the anisotropy of the volume and on non-ergodicity parameters ε H, ε v relative to each direction H,V, the non-ergodicity parameters depending on the volume dimensions along said directions and on the variogram of absolute permeability. - defining a reservoir model comprising a volume of dimensions D H, D V along respectively two distinct directions H,V, - receiving statistical data relative to the volume, comprising: ∘ relative proportions of at least two rock types, wherein each rock type corresponds to a permeability value and respective curves of relative permeability with water saturation of two phases of a fluid within the rock type, one of the phases being water, and ∘ a variogram of absolute permeability defined by correlation lengths L H, L V, along the two directions H,V, and, ∘ relative proportions of at least two rock types, wherein each rock type corresponds to a permeability value and respective curves of relative permeability with water saturation of two phases of a fluid within the rock type, one of the phases being water, and ∘ a variogram of absolute permeability defined by correlation lengths L H, L V, along the two directions H,V, and, - computing, equivalent relative permeability values of a phase of the fluid within the volume, comprising: ∘ at least an equivalent relative permeability value (Kr eq,H) according to the first direction H, and ∘ at least an equivalent relative permeability value (Kr eq,V) according to the second direction V, characterized in that each equivalent relative permeability value of a phase of the fluid according to a direction d chosen among H,V is computed based on relative permeability values of the phase of the fluid within each rock type, and on a coefficient depending on the anisotropy of the volume and on non-ergodicity parameters ε H, ε v relative to each direction H,V, the non-ergodicity parameters depending on the volume dimensions along said directions and on the variogram of absolute permeability. ∘ at least an equivalent relative permeability value (Kr eq,H) according to the first direction H, and ∘ at least an equivalent relative permeability value (Kr eq,V) according to the second direction V,