We study the relation between J-anti-invariant 2-forms and pseudoholomorphic curves in this paper. We show the zero set of a closed J-anti-invariant 2-form on an almost complex 4-manifold supports a J-holomorphic subvariety in the canonical class. This confirms a conjecture of Draghici-Li-Zhang. A higher dimensional analogue is established. We also show the dimension of closed J-anti-invariant 2forms on an almost complex 4-manifold is a birational invariant, in the sense that it is invariant under degree one pseudoholomorphic maps.