The compound gear planetary reducer has the advantage of maintaining high back drive mobility and power transmission efficiency to produce high deceleration ratios in simple structures. In this study, the parameter design problem of the reducer was transformed into a general optimization problem to obtain an optimal solution using the sequential least squares programming (SLSQP) technique, a nonlinear optimization technique. However, the SLSQP method showed advantages only in planetary friction drives. Thus, the optimization was performed based on the module value of the gear to derive optimal parameters of the 3K-type planetary reducer. An optimal design program based on the graphical user interface was developed to simplify its use.