This paper introduces the semantic of Multiparty Joint Authentication (MJA) into the authentication service, which is to find simpli fied or optimal authentication solutions for all involved principals as a whole instead of to authenticate each pair of principals in turn. MJA is designed to support multiparty security contexts in grids with a speci fied, understood level of confidence and reduce the time cost of mutual authentications. Graph theory model is employed to define MJA, and analyze its mathematical properties. Two algorithms to find an n-principal, n-order MJA solution are also presented and proved.