Natural fibres such as flax and hemp fibres are mainly used in the textile industry, but have outstanding mechanical properties. These natural fibres have a great potential as reinforcement in cementitious composites, as an alternative to synthetic microfibres. The multiple cracking capacity of cementitious composites reinforced with natural fibres, however, is inferior due to the hydrophilicity, and the fibres may degrade in alkaline environments. Consequently, a proper mixture design and chemical treatments were investigated to improve fibre characteristics. The application of flax and hemp fibres in cementitious composites was examined, with a focus on inducing multiple cracking under tensile stresses. The mechanical properties of the natural fibres, as well as of the cementitious composites and the degradation of the natural fibres in alkaline environments were studied. Multiple cracking was achieved and further improvements were made by chemically treating the fibres. Mercerization with 2 m% concentration sodium hydroxide (NaOH) resulted in optimal multiple cracking. This multiple cracking resulted in small cracks widths, which allowed optimal autogenous healing when exposed to wet/dry-cycles. Natural fibres were thus found to be a valid eco-friendly alternative to synthetic microfibres.