Objective: The aim of this study was to evaluate implant screw loosening using resonance frequency (RF) analysis with triaxial piezoelectric pick-up in vitro and in vivo. Methods: For the in vitro experiment, a titanium implant was inserted into a mandibular model. The abutment screws were tightened to 10 N torque and loosened from 0 to 90° for RF measurement using 13 different screw conditions. In the in vivo experiment, three titanium implants were installed in each tibia of a New Zealand white rabbit, and the RF values were recorded after 8 weeks. For the RF analyses, a small 3D accelerometer was mounted rigidly onto the implant abutment, and impulsive vibration was directly applied to the abutment to vibrate the implant in a direction perpendicular to the tibia and implant (x-axis). The y-axis was defined as parallel to the tibia. The RF values of the x- and y-axis directions (RF-X and RF-Y) were used for evaluation. Results: The RF values significantly decreased according to the degree of screw loosening (p &lt
0.05, ANOVA). In the in vitro model, RF-Y with x- and y-axis vibrations (RF-Yx and RF-Yy) significantly differed from the initial value at 10 and 15°, respectively (p &lt
0.05, Dunnett’s test). In the in vivo experiment, the RF-Yy significantly differed from the initial value between 5 and 20° (p &lt
0.05). Conclusion: The results suggest that RF analysis with triaxial piezoelectric pick-up can be used to detect implant screw loosening. Clinical relevance: RF analysis with the triaxial piezoelectric pick-up can be used to detect screw loosening after mounting the superstructure.