Let [see formula in PDF] be the finite field, [see formula in PDF], with [see formula in PDF] being a prime and [see formula in PDF] being a positive integer. Let [see formula in PDF] be the multiplicative group of [see formula in PDF], that is [see formula in PDF]. In this paper, by using the Jacobi sums and an analog of Hasse-Davenport theorem, an explicit formula for the number of solutions of cubic diagonal equation [see formula in PDF] over [see formula in PDF] is given, where [see formula in PDF] and [see formula in PDF]. This extends earlier results.