The sample size determination is an important issue for planning research. However, limitations in size have seldom been discussed in the literature. Thus, how to allocate participants into different treatment groups to achieve the desired power is a practical issue that still needs to be addressed when one group size is fixed. The authors focused on the sample size determination for the second group for the two-sample K. Yuen's (1974) trimmed mean test in the condition of heterogeneous variances. The results show that the sample size needed is less than that of the traditional B.L. Welch's test (1938), especially for nonnormal distributions. Simulation results also demonstrate the accuracy of the proposed formula in terms of Type I error and statistical power. (Contains 3 tables.)