Highlights Keywords: Implement, Inertial parameter, Lateral stability, Moment of inertia, Rollover, Tractor.Static lateral stability of agricultural tractors with mounted rotary tillers was analyzed. Keywords: Implement, Inertial parameter, Lateral stability, Moment of inertia, Rollover, Tractor.The mounted implement increased static lateral stability of tractors in phase I rollover but decreased static lateral stability in phase II rollover. Keywords: Implement, Inertial parameter, Lateral stability, Moment of inertia, Rollover, Tractor.A mounted implement may significantly reduce the static lateral stability of agricultural tractors and increase the risk of tractor side overturn. Keywords: Implement, Inertial parameter, Lateral stability, Moment of inertia, Rollover, Tractor. ABSTRACT. Each year, many fatalities result from rollovers of agricultural tractors in Japan. In addition to rollover protective structures (ROPS) and seat belts, a warning device that alerts the operator of impending rollover based on the tractor stability index is a measure used to prevent rollovers. The stability index requires inertial parameters, which have been measured only for the single body of the tractor, to calculate the warning threshold. In this study, the center of gravity (CoG) and lateral stability angles of three agricultural tractors were measured, and lateral stability angles were also calculated and compared with measured values for three tractor-tiller combinations to analyze the effect of the attached implement on the tractor stability as well as to verify the accuracy of the calculation methods. The roll moment of inertia (RMI) was also measured for two tractors and two rotary tillers, and RMI values for tractor-tiller combinations were calculated. The measurement and calculation results show that the attached implement increased the lateral stability angle of tractors in phase I rollover and decreased the lateral stability angle in phase II rollover, and for a certain tractor-tiller combination, there was no transition from phase I to phase II rollover. The difference between the measured and calculated lateral stability angles in phase I ranged from -3.5° to 2.5°, while that in phase II ranged from 0.2° to 5.2°. The RMI about the longitudinal axis through the CoG was 203 and 433 kg m-2 for tractors A and B, respectively, and 52 and 94 kg m-2 for rotary tillers D and F, respectively. The calculated RMI values were 265 and 540 kg m-2 for tractor-tiller combinations A-D and B-E, respectively. Keywords: Implement, Inertial parameter, Lateral stability, Moment of inertia, Rollover, Tractor.