A series of experimental methods including 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide (MTT) test, alkaline phosphatase (ALP) activity measurement, oil red O stain and measurement, mineralized function and quantitive real time RT-PCR (qRT-PCR) were employed to assess the effects of Er3+ on the proliferation, differentiation and mineralization function of primary osteoblasts (OBs) in vitro at cell and molecular levels. The results indicated that Er3+ inhibited the proliferation of OBs at a concentration of 1x10(-7) mol/L, but had no effect at other concentrations. Er3+ inhibited the differentiation of OBs at concentrations of 1x10(-8), 1x10(-7), and 1x10(-6) mol/L, but had no effect at a higher concentration of 1x10(-5) mol/L. Er3+ had no effect on the transdifferentiation of OBs at tested concentrations. Er3+ inhibited the mineralization function of OBs at concentrations of 1x10(-7), 1x10(-6), and 1x10(-5) mol/L, but had no effect at a lower concentration of 1x10(-8) mol/L. The expression of the mRNA for runt-related transcription factor 2 (RUNX-2) and peroxisome proliferators activated receptor gamma (PPAR-gamma) was down-regulated in the presence of 1x10(-6) mol/L Er3+. These findings suggested that Er3+ might have negative effect on bone metabolism.