This chapter provides a detailed protocol for studying the effects of carbon nanotubes (CNTs) on the proliferation, differentiation, adipocytic transdifferentiation, and mineralization of primary osteoblasts. SWNTs, DWNTs, and MWNTs with the same mean length and various diameters were shown to reduce the viability of osteoblasts and inhibit the adipocytic transdifferentiation in both time- and dose-dependent manners. The order of inhibition effect is SWNTs > DWNTs > MWNTs. CNTs were found to inhibit the formation of mineralized nodules greatly and dose-dependently during the final stage of osteoblast differentiation, causing a 50% decrease in the formation of mineralized nodules at the concentration of 50 microg/mL. The expression of important proteins such as Runx-2 and Col-I in osteoblasts was also greatly inhibited by the CNTs. TEM results revealed that the effects on cellular behavior may be exerted by the CNTs from in- and outside of the cells.