Fibre orientation is an essential factor governing the mechanical properties of composite materials. This study proposes an original method based on gray-level granulometry to analyse the fibre orientation distribution (FOD) of synthetic and natural fibre reinforcements aiming composite applications. An orientation maps is computed from SEM images and frequency of fibre orientation is graphically illustrated for each angular direction. First, glass fibre nonwoven and unidirectional preforms were analysed as a model to validate the method before testing their flax fibre counterparts. Differences in structural organisations were found between flax and glass fibre reinforcement FOD due to the specific structure and mechanical behaviour of plant fibres but also to the preform manufacturing process. Promising results were obtained confirming the reliability of this novel numerical method for fibre orientation determination.