Let f ( k 1 , … , k m ) be the minimal value of size of all possible unextendible product bases in the tensor product space ⊗ i = 1 m C k i . We have trivial lower bounds n ( k 1 , … , k m ) = ∑ i = 1 m ( k i - 1 ) + 1 and upper bound k 1 ⋯ k m . Alon and Lovász determined all cases such that f ( k 1 , … , k m ) = n ( k 1 , … , k m ) . In this paper we determine all cases such that f ( k 1 , … , k m ) = k 1 ⋯ k m by presenting a sharper upper bound. We also determine several cases such that f ( k 1 , … , k m ) = n ( k 1 , … , k m ) + 1 by using a result on 1-factorization of complete graphs.