We investigate the reduction ofthe group velocity propagation resulting from the steep change ofthe refractiveindex by the coherent population oscillation in erbium.doped optical fibre.The largest delay is measured to beabout 8.75ms corresponding to a group index of 1.312 x 106.The time delay or advancement depends on theDump intensity.Influences 0f the ion density on the fractional delay and the group velocity reduction oflightpropagation are studied.Based on our discussion the optimization parameters should be selected in order toobtain more appropriate time delay and the slowdown ofgroup velocity.