The effect of the intraband electronic relaxation on the relaxation oscillation of the light output from the injection laser modulated directly is examined theoretically. Assuming the k-selection rule, carriers are devided into two parts; that is, carriers which contribute to the laser gain directly, and the rest. The intraband electronic relaxation is taken into account as the coupling of the two parts of the carriers in rate equations. As the result of the small signal analysis of rate equations, the height of the resonance-like" peak in the frequency response of the direct modulation depends on the electronic relaxation time remarkably in the range from 10-13 to 10-12 s. The intraband electronic relaxation possibly concerns with the suppression of the relaxation oscillation of the light output from the GaAs injection lasers.