Abstract: This article presents theoretical analysis of the grabing force of a log grapple and the force function acted on the grapple's jaw through experiments, it creats the grabbing force formulae under two different working conditions (grabbing compact logs pile and bundle-like logs pile). The tests prove that the formulae are acceptable and three kinds of force functions acted on jaw are developed in the paper. The tests also prove that one of the force functions acted on jaw is parabola distribution during grabbing compact logs pile, the other is sine curve distribution during grabbing bundle-like logs pile.