Hemocytes, the cellular component of invertebrate hemolymph, are essential for invertebrate immunity, but the hematopoiesis and regulation mechanism are still largely unknown. In the present study, a conserved hematopoietic transcription factor Cg-GATA2/3 was identified in Pacific oyster Crassotrea gigas, which was evolutionarily close to the vertebrate GATA1/2/3. Cg-GATA2/3 was mainly distributed in the immune organs, such as gill, hemocytes, and mantle. After Cg-GATA2/3 was interferenced by dsRNA, the mRNA expressions of hemocytes specific gene (EcSOD) and hematopoietic transcription factor (C-Myb) were all significant down-regulated, and the hemocyte renewal rates also decreased both in hemolymph and gill. During the larval developmental stages, the mRNA transcripts of Cg-GATA2/3 increased immediately after fertilization and kept a high level during blastula and early trochophore larvae stage (4-10 hpf, hours post fertilization), then decreased sharply in early D-veliger larvae stage (15 hpf). Whole-mount immunofluorescence assay further revealed that the abundant immunoreactivity of Cg-GATA2/3 was distributed in the whole body of blastula and gastrula embryos, while specialized gradually to a ring structure around the dorsal region in trochophore larvae. In the D-veliger and umbo larvae, scattered positive signals appeared in the specific sinus structure on the dorsal side and velum region. These results demonstrated that Cg-GATA2/3 was a hematopoietic lineage-specific transcription factor to regulate the hemocyte production, and it could also be used as hematopoietic specific marker to trace potential developmental events of hematopoiesis during ontogenesis of oyster.