A class of isothermal throttling process with generalized mass transfer law is investigated, and the optimality condition for the minimum mass entransy dissipation of the process is obtained by applying optimal control theory. The results for special cases with mass transfer laws g∝(Δp)m and g∝Δ(μ) are further obtained based on the general optimization result, and the obtained results are also compared with other mass transfer strategies of the minimum entropy generation, constant pressure ratio and constant pressure difference. Numerical examples for the cases with the mass transfer laws g∝(Δp)1/2, g∝Δ(p) and g∝Δ(μ) are also provided. The results obtained herein can provide some theoretical guidelines for the optimal design and operation of real throttling processes and devices.