An asymptotic analysis for terminal velocity of a rigid sphere has been made employing the Oseen law for drag force. On the basis of the asymptotic analysis, our previous result (a four-parameter approximation) for terminal velocity of rigid spheres (Nguyen et al., 1994) has been improved. The terminal velocity can be adequately described by a three-parameter model given by Eq. (21) in our present paper. The improved formula: (1) does not numerically differ from our previous model; (2) is simpler than the previous one since it is only a three-parameter approximation; and (3) provides a unified and useful approximation for terminal velocity of solid particles in the modelling of the flotation process.