Using high resolution optical microscopy coupled with image analysis software and statistical methods, fibre length and aspect ratio distributions in polypropylene composites were characterized. Three types of fibres, flax, sisal and wheat straw, were studied. Number and surface weighted distributions were used to demonstrate the presence and amount of elementary fibres, fibre bundles and particles. A large number of small particles that are usually not taken into account were found. The obtained fibre length and aspect ratio distributions were successfully fitted by a two-parameter Weibull model. No significant effect of fibre concentration on the length and aspect ratio distributions was detected for 20% and 40% flax-based composite. Fibre type was found to be a very important parameter influencing size and shape distributions: flax is broken into long elementary fibres, sisal-based composite contains non-dispatched bundles and elementary fibres and wheat straw-based composite has bundles and large and small particles.