Bed material load is composed of bed-load motion and suspension interacted each other, and the interaction is represented by transition from saltation to suspension, of which quantitative evaluation must support a reasonable description of bed material load transport process. The trajectory of a saltation particle is rather deterministic though it fluctuates due to turbulence. When such a shift of particle's position from the deterministic trajectory is larger than a threshold value, it brings a transition from saltation to suspension. Such an event is explained by a kind of logistic equation deduced from the equation of particle's motion. The probability that the shift of the particle's position from the deterministic trajectory overcomes the threshold value expresses the probability of transition. The probability density per unit time for a bed-load particle to turn into suspension is reasonably calculated and a convenient mathematical expression of the relation between bed shear stress and it is proposed.