PURPOSE: The aim of this retrospective study was to evaluate the influence of implant diameter, length and placement to implant stability. MATERIALS AND METHODS: Total 90 implants (US II plus(TM), Osstem co, Busan, Korea) of 72 patients were determined as experimental samples. The factors of diameters(o 4 mm, o 5 mm), lengths (10 mm, 11.5 mm, 13 mm), and implant placement (maxilla, mandible) were analyzed. The stability of the implants was measured by resonance frequency analysis (RFA) at the time of implant placement and impression taking. The difference of ISQ values according to patient's gender was evaluated by Independent t-test. ISQ values were compared between implant diameter, length and placement using one-way ANOVA and Tukey HSD test (alpha=.05). To compare ISQ values between at the time of surgery and impression taking, paired t-tests were used (alpha=.05). RESULTS: The change of implant length did not show significant different on the ISQ value (P>.05). However, 5 mm diameter implants had higher ISQ values than 4 mm diameter implants (P<.05). Implants placed on the mandible showed significantly higher ISQ values than on the maxilla (P<.05). CONCLUSION: In order to increase implant stability, it is better to select the wider implant, and implants placed on mandible are possible to get higher stability than maxilla. ISQ values at impression taking showed higher implant stability than ISQ values at implant placement, it means that RFA is clinically effective method to evaluate the change of implant stability through the osseointegration. The consideration of the factors which may affect to the implant stability will help to determine the time of load applying and increase the implant success rate.