In this paper, we derive a relation between the wavelength chirp and carrier temperature in semiconductor lasers, The coefficient relating the change in carrier temperature and chirp is expressed in terms of the temperature derivative of the optical gain, and two parameters describing the variation of refractive index produced by the variation of optical gain due to change of carrier quasi-Fermi level separation or carrier temperature, We have measured these parameters for MQW InGaAsP lasers. Using this data, we estimated the rate of the temperature increase with current above threshold ire these devices, which is 0.13 K/mA.