Abstract: The gain optical guiding effects in a chemical oxygeniodine laser (COIL) are found and studied for the first time. The nonuniform distribution of gain in one transverse direction is assumed to be caused by nonuniform injection of the secondary flow. Simple expressions are derived to predict the cavity singlet delta oxygen and intensity profile using the method of Hager et al. Our calculations show that the gain and optical guiding effects have an important influence on the transverse shape of the optical field. The laser beam has a trend to produce similar shape of the gain, which results in some effects such as the offset of the optical beam and the reduce of the output power. The optical guiding can be observed in experiment by measuring the transverse shape of the laser beam.