Knee osteoarthritis (KOA) is a common joint disease of the lower limbs. Its progression reduces the patients’ quality of life. Varus thrust (VT), one of the abnormal gait patterns of KOA patients, is considered as an effective index for assessing KOA. Hence, several studies have assessed VT using various measurement methods. Since VT is the momentary lateral knee motion that increases knee varus angle and moment, optical motion capture system is widely used. However, optical motion capture system has some disadvantages in clinical usage, such as high cost, requirement of technical skills, and time-consuming attachment process. Recently, inertial measurement units (IMU) have emerged as a measurement system instead of optical motion capture systems. IMU-based method is regarded as more suitable than optical motion capture systems for VT assessment because of its simplicity. This study aimed to assess the gait of KOA patients using IMUs and to quantitatively evaluate VT based on disease progression. For this purpose, we recruited 7 healthy participants and 15 KOA patients. Subsequently, their knees were classified into 3 progression groups: 14 healthy (grade 0) knees, 9 grade 3 knees, and 14 grade 4 knees. As a gait test, all the participants wore IMUs positioned at the trunk, both thighs, and both shanks, and traversed a 10-m walkway. VT was considered as the first peak value of the mediolateral acceleration and the varus–valgus angular velocity data, which were collected via the IMUs at both shanks. Thereafter, these acquired data were compared and evaluated among the three progression groups. The results indicate that both peak values were significantly greater in the KOA patients than in the healthy subjects. Moreover, there was a significant positive correlation between the two peak values. Thus, this study is expected to contribute toward early detection of KOA.