Two models of gain and thermal guiding effects were derived. In the first one the complex ABCD matrix for a crystal under gain and thermal guiding was applied to describe the operation of microchip near threshold. In the second one, the simple iterative procedure was proposed to calculate effective fundamental mode parameters of a cavity under thermal and gain guiding for given bare cavity ABCD matrix and pumping parameters, including gain saturation, passive cavity losses and reabsorption ones. The influence of gain guiding effects causes changes of waist width in the range up to 50% comparing to expectations derived from thermal guiding theory. Application of such method for resonators of passively Q-switched lasers was developed. Results of calculations for microchips were verified with experiment.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.