Systems and methods for estimating tissue parameters, including mass of tissueto betreated and a thermal resistance scale factor between the tissue and anelectrode of an energydelivery device, are disclosed. The method includes sensing tissuetemperatures, estimating amass of the tissue and a thermal resistance scale factor between the tissueand an electrode, andcontrolling an electrosurgical generator based on the estimated mass and theestimated thermalresistance scale factor. The method may be performed iteratively and non-iteratively. Theiterative method may employ a gradient descent algorithm that iteratively addsa derivative stepto the estimates of the mass and thermal resistance scale factor until acondition is met. The non-iterative method includes selecting maximum and minimum temperaturedifferences andestimating the mass and the thermal resistance scale factor based on apredetermined reductionpoint from the maximum temperature difference to the minimum temperaturedifference.
