In this paper,we consider the following semilinear elliptic equation:{-△u =h(x,u) in Ω,u ≥ 0 on Ω,where Ω is an exterior domain in RN with N ≥3,h:Ω × R+ → R is a measurable function,and derive optimal nonexistence results of positive supersolutions.Our argument is based on a nonexistence result of positive supersolutions of a linear elliptic problem with Hardy potential.We also establish sharp nonexistence results of positive supersolutions to an elliptic system.