The behavior of osteoblasts, bone-forming cells, on air plasma-treated poly(L-lactic acid) was observed. Osteoblasts were isolated from calvaria of neonatal Sprague Dawley rat by an enzymatic digestive process. A confluent number of osteoblasts (4x10⁴cells/cm²) was suspended with Dulbecco''s modified eagle medium supplemented with 10% fetal bovine serum, and cultured on the polystyrene culture dish coated with PLLA or air plasma-treated PLLA. The attached cells were counted using a hemocytometer to evaluate cell adhesion and growth. Cell morphology was observed using a scanning electron microscope(SEM). Calcium contents deposited on the materials after cell culture were measured using an atomic absorption spectrophotometer(AAS) to evaluate mineralization by osteoblasts. Cells were more spreaded on the plasma-treated PLLA than on the control. Cells were more proliferated on the plasma-treated PLLA than on the control when measured the number of cells after 72 h. In addition, calcium content on the plasma-treated PLLA were higher than that on the control.