A method for estimating strain in a semiconductor laser chip arising from bonding to foreign materials. The method is based on measurements of dependence of lasing wavelengths on duty ratios of pulsed operation with a constant peak current. The wavelength at zero duty ratio (&lgr;0) is extrapolated from this dependence, from which the strain is estimated. Estimated strain from this method is in good agreement with result obtained from the change of polarization ratio, transverse‐magnetic to transverse‐electric light outputs. The strain arising from packaging affects not only lasing wavelength but also operating characteristics such as threshold current. Moreover, thermal resistance of laser diodes can be obtained from this dependence of lasing wavelengths on duty ratios. ©1995 American Institute of Physics.