Oxidative stress has been reported to influence osteoporosis occurring at the older age. Osteoblasts play an important role in bone remodeling. The secondary metabolites of plants including phenols and flavonoids show the abilities to scavenge free radicals. We evaluated eight extracts obtained from different parts of Taiwanese endemic plants by using chemical analysis and antioxidant assays and establishing two in vitro models for osteogenic activities. Chemical analysis was evaluated by determining total phenolic and flavonoid contents. The antioxidant activities were determined by scavenging DPPH and ABTS radicals. Osteogenic activities were evaluated in osteosarcoma cell lines (MG-63) and primary human osteoblasts (HOb) by alkaline phosphatase (ALP) activity, an early-stage biomarker, and mineralization, a late-stage biomarker. Results exhibited that four extracts, ESS, BBW, TFL, and TFR, demonstrated higher total phenolic contents (more than 100 mg gallic acid equivalent/g of extract). ESS, BBW, TUW, and AKR contained higher total flavonoid contents (more than 100 mg rutin equivalent/g of extract). Six extracts, ESS, BBW, TFL, TFS, TFR, and TUW, significantly scavenged DPPH and ABTS radicals by more than 50%. None of the extracts showed cytotoxicity in MG-63 and HOb cells. Four extracts, ESS, BBW, TFL, and TFR, significantly enhanced ALP activity, and five extracts, ESS, BBW, TFL, AKR, and WTL, increased mineralization in MG-63 cells. In addition, ESS, TFL, TFS, and WTL significantly enhanced ALP activity in HOb cells. From these results, we deduce that ESS and TFL contained higher total phenolic and flavonoid contents, antioxidant and osteogenic activities were also higher in these two plants. Our in vitro models are well established for screening the agents with the osteogenic activity. This will provide us with the potent Taiwanese endemic plants that can serve as the sources of preventive agent for oxidative stress and osteoporosis.