The femur of chicken embryo in experimental fluorosis was observed under light and electron microscope, and studied by bone histomorphometric method. The results showed that most of osteoblasts were cuboid and crowded together on the surface of bone trabecula in experimental group. The osteoid was increased and mineralized bone was diminished. The bone reformation indexes were higher significantly and the bone resorption indexes did not show differences between the two groups. The cross striations of type Ⅰ collagen fibrils in osteoid layers of experimental group were indistinct and bone lamella were jumble array. Our results suggest that though fluoride has a stimulating action on the embryonic osteoblasts and enhanced the bone formation, the osteoblasts produced abnormal collagen fibrils which caused disorder of osteoid mineralization and led to osteomalacia.