Abstract: In this paper a theory is given for the interpretation of drawdown and buildup tests,which are given to an individual layer of a multilayer reservoir with crossflow.The crossflow behavior and its effect on drawdown and buildup curves are discussed.It is found that both drawdown and buildup curves have two straight lines with a transition period bet-ween them.The first straight line period is the period when the theory of well test for a single layered reservoir is effective for an indivi-dual layer test in a multilayer reservoir with crossflow.The first strai-ghtline determines the kh product and the skin factor of the test layer.The second straight line determines the total kh product of the reservoir.The vertical permeabilities of the shales between layers can be determined by two method,using the crosspoint of the two straight lines of drawdown or buildup carves and the steady wellbore pressure diffe-rences between the test layer and the shut down layers.