A mathematical model is obtained for the description of ultrafast stimulated Raman scattehog in optical fibers with slowly decreasing dispersion. Numerical simulations show that, under identical pump conditinns, Raman pulse generated from this kind of fibers is shorter with a higher ark power than that generated from conventional fibers. This means that the Raman threShold of fibers with slowly decreasing dispersion may be lower than that of conventional fibers. Given pump conditions, we found that the highest peak power and narrowest width of the Raman pulse correspond to an optimal decrement velocity of the fiber dispersion.