The suppressor of cytokine signaling-1 (SOCS-1) is a cytokine-inducible intracellular molecule that inhibits excessive activation of the JAK-STAT-mediated signal cascade initiated by various stimuli. The smaller size of SOCS-1 knockout (KO) mice suggests the presence of skeletal abnormality caused by the disruption of the regulatory system in JAK-STAT signaling. In addition to macroscopic examination, peripheral quantitative computed tomography (pQCT), bone histomorphometrical analysis, and in situ hybridization were used to examine the skeletal properties of SOCS-1 KO mice. Moreover, differentiation of primary cultured osteoblasts was investigated. Distinct phosphorylation of STAT1 was detected in the SOCS-1 KO calvarial cells but was hardly detectable in wild-type (WT) mice. Undercalcified areas in the skulls and sternum, as well as comparatively thinner calcified areas of cortical bone, were found in SOCS-1 KO mice. pQCT analysis showed a marked decrease in salt content, whereas the mineralization activity of primary cultured calvarial cells strongly suggested significant impairment in osteoblasts of SOCS-1 KO mice. In situ hybridization analysis demonstrated that these mice expressed the early markers [type I collagen (COL-1) and osteonectin (ON)] and the mid-marker [osteopontin (OP)] at levels comparable with those seen in WT mice. However, a dramatic decrease was observed in the expression level of the late marker [osteocalcin (OC)] of osteoblasts. Our findings thus demonstrate that SOCS-1 regulates osteoblast differentiation in the later stage.