1. A method of instant estimation of the rotor inertia of a load motor used in a servo control system with a controlled object, the method comprising the steps of: referring to a model motor to determine an input signal of an adaptive filter according to the current signal of the model motor; determining a desired signal of an adaptive filter according to the error between a drive motor current signal and the model motor current signal; obtaining a plurality of delay signals according to the input signal and multiplying the delay signals by the corresponding weights to output a weighted output signal; determining the error between the weighted output signal and the desired signal; and determining by the adaptive filter the rotor inertia of the servo control system according to the input signal and the desired signal using the Least Mean Square (LMS) Rule.