The effects of gain guiding and thermal guiding on the cavity mode formation for quasi-three-level microchip lasers are investigated. A parameter describing the dominant guiding mechanisms at the lasing threshold is defined. Paraxial wave propagation equation for the field envelope in the gain medium with thermal-induced refractive index profile and gain profile is derived. A comparison between the calculated results and those of experiments reveals that this model is quite reasonable and applicable not only to the Yb:YAG microchip laser, but also to other quasi-three-level microchip lasers.