The impact of thermal induced temperature gradient on the properties of transversal modes propagating in fiber lasers is carefully investigated. The thermal distribution in the cross section of fiber laser is figured out through a predigested heat conductive model, and then, the properties of transversal modes are analyzed by using multiphysics software under different operating conditions or parameters of fiber. It is found that all the transversal modes have a trend of constriction to the core of fiber with rising power level, and this discovery may give one of the explanations how the high order modes appear in single mode high power fiber lasers. As well, when the absorption coefficient or the refractive index-temperature coefficient is growing, the transversal modes also present a constriction trend.