Dynamic resistance of solar cells and modules have been determined from a dark IV characteristic curve. In the determination, it is often assumed that series resistance R-S is small and the shunt resistance R-sh very large, their effects can be neglected. The resultant dynamic resistance can be called the external dynamic resistance R-D.R-D is normally taken to be the slope of the IV characteristic of a cell or a module. We present in this paper a new method to determine an internal dynamic resistance R-d of a photovoltaic module based on one illuminated IV curve, taken into account finite series and shunt resistance. In the experiment, we measure illuminated and dark characteristics of a 4.5 Wp commercial X-Si solar module (9 V, 500 mA) at 25-27 degrees C, and calculate R-d and R-D. We see that Rd and RD can be markedly different. Comparison is also made on the series resistance RS and shunt resistance Rsh determined from single IV curve and two IV curve methods. (c) 2006 Elsevier B.V. All rights reserved.