The ubiquitin ligase constitutively photomorphogenic 1 (COP1) is involved in many biological responses in mammalian cells, but its role in tumorigenesis remains unclear. Here we show that COP1 is a ubiquitin ligase for the tumor suppressor CCAAT/enhancer-binding protein (C/EBPα) and promotes its degradation in vivo, thereby blocking myeloid differentiation of hematopoietic cells for tumorigenesis. In this process, mammalian homolog of Tribbles, Trib1, which contains a COP1-binding motif, is essential for down-regulation of C/EBPα expression. Murine bone marrow transplantation experiments showed that coexpression of COP1 accelerates development of acute myeloid leukemia induced by Trib1, which pathologically resembles that of p42C/EBPα-deficient mice. Interestingly, coexpression of ligase activity-deficient COP1 mutant abrogated Trib1-induced leukemogenesis. These results indicate that COP1 and Trib1 act as an oncoprotein complex functioning upstream of C/EBPα, and its ligase activity is crucial for leukemogenesis.