In recent years, interest in smart and green vehicles has increased. There are many cases where engine net torque related control is performed when building smart and green car systems. Speed tracking control in autonomous driving, or optimal transmission shift control is an example. The engine net torque is the sum of the engine indicated torque, accessory load torque, and fictional torque, and the starter motor torque in the case of a parallel hybrid vehicle. However, the estimation error of these torque items can cause the estimation error of the engine net torque. In this paper, a compensation method for the slowly varying uncertainty of the engine net torque in a parallel hybrid vehicle using a multiplicative constant is proposed. The adaptation of the multiplicative constant is conducted using the amount of change in the engine net torque estimated in the backward direction of the driveline. The proposed algorithm is verified based on production vehicle data.