The objective of this study was to analyse bio-fibre comminution energy and particle size classification as the first step towards converting cellulose based agricultural residues into environment-friendly composite products. Bio-fibres used in the study were water retted, chemically modified and oven dried to remove any trace of moisture. The bio-fibres comminution was performed in a laboratory-scale knife mill at varying mill speed, and chemical treatment concentrations. The particle size classification was aided by mechanical sieving process. Nonlinear Least Square method using the cftool based on Gauss-Newton method of iteration available in MATLAB 2007b was used to fit the experimental data to Rosin-Rammler-Bennett’s (RRB) and Gaudin-Schuhmann’s (GS) size distribution models. Energy consumption was estimated via Kick’s, Rittinger’s and Bond’s models. The result showed that energy requirement for bio-fibre comminution increases with decreasing particle size, with coir fibres requiring the highest energy when compared with banana fibres. As the mill speed increases from 1400 rpm to 2100 rpm the energy consumed in comminution banana fibre increases from 357.46 J/g to 369.38 J/g for 2% NaOH treatment with the kick’s constant increasing from 52.97 J/g to 57.86 J/g for untreated banana fibres. Whereas for coir fibre, the energy consumed in comminution increases from 382.14 J/g to 411.07 J/g for 2% NaOH treatment with the Kick’s constant increasing from 57.25 J/g to 61.83 J/g for untreated coir fibres. Bio-fibre energy consumption was best described by Kick’s model with R2 > 0.910 and RRB model captured the bio-fibres size distribution better than GS model with higher R2 and lower value of SSE and RMSE. Scanning electron microscopy revealed that the surface of the fibres had changed, becoming rougher as a result of chemical modification. Findings also indicated that comminution of bio-fibres with high chemical concentration results in less fine sizes with median size (S50) ranging from 524 μm–1476 μm for banana fibres and 644 μm–660 μm for coir fibres. The data obtained at different speeds of comminution and from various bio-fibre treatment conditions might guide the selection of initial fibre size and other processing parameters to better exploit bio-fibres potential in particulate fibre reinforced composites products design.