The paper presents the results of a study on the influence of moisture content of plant biomass (Virginia mallow) and of unit piston pressure on the parameters of briquetting (compaction), on the compaction ability of raw material, and on the quality of obtained agglomerates. Compaction of the raw material was carried out using a Zwick testing machine type ZO2O/TN2S and a closed die compression assembly with a diameter of 15 mm. Material moisture varied from 8 to 18%. Compaction was performed for three maximum levels of unit pressure of the piston on the material – 57, 85 and 113 MPa. It was found that an increase in the moisture content caused an increase of the density of the material in the chamber and of the expansion of the agglomerate, while the agglomerate density and the work of compaction decreased. The increase in raw material moisture content improved the compaction ability and worsened the quality of the agglomerates in terms of their strength. With increasing piston pressure there was an increase of the density of the agglomerate, compaction work and mechanical strength of the agglomerate.