This paper studies intersections of principal blocks of a finite group with respect to different primes. We first define the block graph of a finite group G, whose vertices are the prime divisors of |G| and there is an edge between two vertices p = q if and only if the principal p-and q-blocks of G have a nontrivial common complex irreducible character of G. Then we determine the block graphs of finite simple groups, which turn out to be complete except those of J 1 and J 4 . Also, we determine exactly when the Steinberg character of a finite simple group of Lie type lies in a principal block. Based on the above investigation, we obtain a criterion for the p-solvability of a finite group which in particular leads to an equivalent condition for the solvability of a finite group. Thus, together with two recent results of Bessenrodt and Zhang, the nilpotency, p-nilpotency and solvability of a finite group can be characterized by intersections of principal blocks of some quotient groups.