Abstract: We propose a gate level algorithm for average power estimation in combinational circuit under the unit delay model and the general delay model.The well-known lag-one Markov Chain Model is used to describe the signal transition.Function transition and glitch transition associated with the switching activity of a node in the circuit, are separately calculated by two newly developed calculation models.By constructing the ROBDD only once according to the node logic function,the function transition can be calculated.This model considers the temporal and spatial correlations among signals.Glitch transition can be calculated by the delay difference of all the inputs on a gate and this gate's glitch-generating patterns appearing at the inputs.Experimental result indicates that our algorithm's accuracy is within 10% and the run-time is shorter by an order in comparison with the Monte-Carlo statistical algorithm.