According to the analysis of one-dimensional radial unsteady heat conduction in an infinite medium,a transient line-source method to measure thermal conductivity of blood is provided, which is effective,accurate and easy to be carried out. By means of this method,it takes only a few seconds to accomplish the measurement. This would be a particular advantage for blood since it coagulates very quickly. With a simple apparatus,experiment has been performed and the results match the existing data of blood' s thermal conductivity very well.