In this paper, we propose a new error assessment method for isogeometric analysis of 2D heat conduction problems. The posteriori error estimation is obtained by resolving the isogeometric analysis problem with several k -refinement steps. The main feature of the proposed method is that the resulted error estimation surface has a B-spline form, according to the main idea of isogeometric analysis. Though the error estimation method is expensive, it can be used as an error assessment method for isogeometric analysis. Two comparison examples are presented to show the efficiency of the proposed method.