In this article, we report an independent work of positional cloning and functional characterization of OsJAG gene in rice. The merit of our work is that we used a genuine null mutant, in which the wild-type allele was completely deleted. This allowed us to identify the mutant phenotypes accurately without the interference of residual function of the target gene. OsJAG is an important gene with pleiotropy, expressing almost throughout the plant and acting in both vegetative phase and reproductive phase. But its main and crucial roles are in regulating the development of all floral organs, especially in specifying the identity of stamens. Interestingly, OsJAG does not affect the number of floral organ primordial and so of floral organs in each whorl, suggesting that OsJAG does not influence the initiation of floral organ primordia, but affect the developmental fate of all floral organs after their primordia have initiated. Loss of OsJAG function results in maldevelopment of all floral organs, such as degenerated lemma and palea, elongated lodicules and deformed and sterile pistil. The stamen appears to be more sensitive to the mutation. All the six stamens in a mutant floret were thoroughly transformed into six pistil-like organs developed at the presumptive positions of the stamens in whorl 3.