This study investigated the effect of particle size, pressure and mold diameter on the physical characteristics of rice straw briquettes. The briquettes were prepared utilizing a manually operated piston-press cold densification system and analyzed in terms of initial density, stable density, density ratio, compaction ratio, percentage of volume change, shatter index and energy consumption. Reduction of mold diameter significantly (p < 0.05) improved briquettes stable density and reduced compaction pressure requirement. Particles of < 2.5 mm or 0.1–150 mm, pressure ≥ 27.6 MPa and mold diameter of ≤ 51 mm should be used to prepare cold densified rice straw briquettes of higher stable density and durability. Interestingly, the briquettes of 0.1–150 mm particles showed significantly (p < 0.05) higher shatter index of > 0.90 even at lower pressure at a cost of stable density. The use of sawdust as binding material at 3:1 and 1:1 mixing ratios increased briquettes stable density > 600 kg/m3, improved shatter index significantly (p < 0.05), increased heating value by 6–7.2%, and reduced ash content from 13.61% to 10.3% and 6.93%, respectively. The energy consumed for briquette preparation accounted 5.6–7.5% and 11.1–13% of the energy which was stored in rice straw briquettes for large and small mold respectively.