Let D v,b,k denote the family of all connected block designs with v treatments and b blocks of size k. Let d ∈ D v,b,k . The replication of a treatment is the number of times it appears in the blocks of d. The matrix C(d) = R(d) − 1 k N (d)N (d) ⊤ is called the information matrix of d where N (d) is the incidence matrix of d and R(d) is a diagonal matrix of the replications. Since d is connected, C(d) has v − 1 nonzero eigenvalues µ1(d), . . . , µv−1(d). Let D be the class of all binary designs of D v,b,k . We prove that if there is a design d * ∈ D such that (i) C(d * ) has three distinct eigenvalues, (ii) d * minimizes trace of C(d) 2 over d ∈ D, (iii) d * maximizes the smallest nonzero eigenvalue and the product of the nonzero eigenvalues of C(d) over d ∈ D, then for all p > 0, d * minimizes v−1 i=1 µi(d) −p 1/p over d ∈ D. In the context of optimal design theory, this means that if there is a design d * ∈ D such that its information matrix has three distinct eigenvalues satisfying the condition (ii) above and that d * is E-and D-optimal in D, then d * is Φp-optimal in D for all p > 0. As an application, we demonstrate the Φp-optimality of certain group divisible designs. Our proof is based on the method of KKT conditions in nonlinear programming.