While newly installed infrastructure in several European cities for bicycles rental offer a real alternative for mobility, road relief variation is still a limiting factor for the generalization of their use by all people. Electric bicycles (EBs) are already commercially available but the power assistance strategy is generally unsuitable and is just proportional to the rider effort and does not compensate ground slope. In this paper, an unknown input proportional two integrals observer is designed in order to robustly estimate the environmental disturbance constituted by the gravitational force and frictional force. The proposed estimation approach is shown to be effective through tests on real road slope profile. The estimations is finally used to compensate the disturbance via the electric motor.