A retention theory in sedimentation field-flow fractionation (SdFFF) was developed by exploiting the effective slip boundary condition (BC) that allows a finite velocity for particles to have at the wall, thereby alleviating the limitations set by the no-slip BC constraint bound to the standard retention theory (SRT). This led to an expression for the retention ratio R as R = (R-o + nu*(b))/(R-o + nu*(b)), where R-o is the sterically corrected SRT retention ratio and nu*(b) is the reduced boundary velocity. Then, nu*(b) was modeled as nu*(b) = nu*(b,o) /[1 + (7K*S-o)(1/2)], where S-o is the surfactant (FL-70) concentration and K* is the distribution coefficient associated with the Langmuirian isotherm of the apparent effective mass against S-o. We applied this to study the surfactant effect on the retention behaviors of polystyrene (PS) latex beads of 170-500 nm in diameter. As a result, an empirical relation was found to hold between nu*(b,o) and d(o) (estimated from R-o at S-o = 0) as nu*(b,o) - nu*(o,o)[1 - (d(c)/d(o))], where nu*(o,o) is the asymptotic value of nu*(b,o) in the vanishing d(c)/d(o) limit and d(c) is the cutoff value at which nu*(b,o) would vanish. According to the present approach, the no-slip BC (nu*(b,o) = 0) was predicted to recover when d(o) similar to d(c), and the boundary slip effect could be significant for S-o <= 0.05%, particularly for large latex beads.