Biomass is an excellent renewable energy source for producing energy with low greenhouse gas and low acid gas emissions that can be intensively used as substitute for fossil fuels. Unfortunately, because of its low bulk density, the effective utilization of the biomass is not always practical. Densification process is often required to improve transportation and storage properties and improve the cost of low bulk density feedstocks. The goal of this study was to determine the ability of microwave and microwave/chemical pretreatments to enhance compaction characteristics and densification process of barley straw. In this study, ground barley straw samples were immersed in water, sodium hydroxide and calcium hydroxide solutions with different concentrations and then exposed to microwave at three irradiation powers. The compression characteristics of biomass samples were examined using a single pelleting apparatus with a compression force of 4000 N and relaxation test data were recorded. The chemical composition, bulk and particle densities, and particle size of samples were investigated after pretreatments. The specific energy required to compress and extrude pellets from untreated and microwave or microwave/chemical pretreated barley straw was calculated. The tensile strength of the pellets was also evaluated via the diametral compression test. Chemical composition analysis showed that microwave/chemical pretreatment is significantly able to alter the lignocellulosic structure of biomass materials so that barley straw samples pretreated by microwave/alkali technique had lower cellulose, hemicellulose and lignin contents than samples pretreated by microwave alone or untreated samples. Data analysis also indicated that microwave/chemical pretreatment is significantly effective to enhance pellet density, tensile strength and compaction characteristics of biomass.