In this study, three medicinal and aromatic plants (Foeniculum vulgare, Malva sylvestris Land Thymus vulgaris) were dried in a pilot scale gas engine driven heat pump drier, which was designed, constructed and installed in Ege University, Izmir, Turkey. Drying experiments were performed at an air temperature of 45 degrees C with an air velocity of 1 m/s. In this work, the performance of the drier along with its main components is evaluated using exergy analysis method. The most important component for improving the system efficiency is found to be the gas engine, followed by the exhaust air heat exchanger for the drying system. An exergy loss and flow diagram (the so-called Grassmann diagram) of the whole drying system is also presented to give quantitative information regarding the proportion of the exergy input dissipated in the various system components, while the sustainability index values for the system components are calculated to indicate how sustainability is affected by changing the exergy efficiency of a process. Gas engine, expansion valve and drying ducts account for more than 60% amount of exergy in the system. The exergetic efficiency values are in the range of 77.68-79.21% for the heat pump unit, 39.26-43.24% for the gas engine driven heat pump unit, 81.29-81.56% for the drying chamber and 48.24-51.28% for the overall drying system. (C) 2010 Elsevier Ltd. All rights reserved.