Abstract: I_λ-Optimal criterion is one which minimizes on the average the predictor variance. In this paper, for generalized simplex-centroid design of q-components and n-degrees on the basis of the variance-covariance matrix expanded expression, which is obtained by this author, we propose a way to find I_λ-optimal observation allocations by parfifioning, and give a simple calculation method. A generalized simplex-centroid design example which has five-component and three degree is included to illustrate the calculation method of I_λ-optimal observation allocations.