A comprehensive study of the compression characteristics of ground non-treated and steam exploded barley, canola, oat and wheat straw obtained from three hammer mill screen sizes of 6.4, 3.2 and 1.6 mm at 10% moisture content (wb) was conducted.  Four preset pressures of 31.6, 63.2, 94.7 and 138.9 MPa, were applied using an Instron Testing Machine to compress samples in a cylindrical die.  It was determined that for non-treated and steam exploded straw, pressure and pre-treatment were significant factors affecting the compact density.  Ground steam exploded barley straw at screen sizes of either 3.2 or 1.6 mm produced high density compacts, while ground steam exploded canola, oat and wheat straw at screen sizes of 6.4, 3.2 or 1.6 mm produced high density compacts.  Steam exploded barley straw for 3.2 mm screen size at a pressure of 138.9 MPa produced compacts having 13% higher density and consumed 19% lower total specific energy compared to non-treated straw.  Steam exploded canola straw for 1.6 mm screen size at a pressure of 138.9 MPa produced compacts having 13% higher density and consumed 22% higher total specific energy compared to non-treated straw.  Steam exploded oat straw for 3.2 mm screen size at a pressure of 94.7 MPa produced compacts having 19% higher density and consumed 13% higher total specific energy compared to non-treated straw.  Steam exploded wheat straw for 6.4 mm screen size at a pressure of 138.9 MPa produced compacts having 17% higher density and consumed 17% higher total specific energy compared to non-treated straw.  Three compression models, namely: Jones model, Cooper-Eaton model, and Kawakita-Ludde model were considered to determine the pressure-volume and pressure-density relationship of non-treated and steam exploded straws.  Jones model indicated that the steam exploded straw was more compressible than non-treated straw.  Cooper-Eaton model showed that non-treated and steam exploded straw was primarily densified by the process of particle rearrangement.  Kawakita-Ludde model provided the best fit to the experimental data having R² values of 0.99 for non-treated straw and 1.00 for steam exploded biomass samples.  The steam exploded straw had higher porosity than non-treated straw.  In addition, the steam exploded straw was easier to compress since it had lower yield strength or failure stress values compared to non-treated straw.