Abstract This paper deals with an estimation method of parameter variations of dynamic systems based on the disturbance observer. The disturbance observer is designed to estimate the parameter variations as disturbances. The simple least squares method can extract the parameter variations from the estimated disturbances which include external disturbances and noise. The proposed method is applied to the tyre pressure diagnosis system which can detect underinflated tyres in every situation such as a puncture and natural leaks using wheel speed sensors already provided for anti-lock brake system. Several experimental results are shown to confirm the effectiveness of the method.