The structural potential of plant fibres as reinforcing agents can only be realized when the highest reinforcement efficiency is employed. Hence, aligned plant fibre composites are of interest. However, due to the short length of technical plant fibres, the reinforcement needs to be in the form of staple fibre yarns, which have a twisted structure. Although twist facilitates yarn processability, it has several detrimental effects on the composites produced from such twisted yarn reinforcements; one of which is fibre obliquity and misalignment. This results in a drastic drop in composite mechanical properties. No analytical model currently exists to accurately predict the effect of yarn twist on aligned plant fibre composite tensile strength. In this paper, a novel mathematical model is developed. The model is based on (i) a modified rule of mixtures for plant fibre composites, (ii) well-defined structure-property relationships in an idealised twisted staple fibre yarn and (iii) the Krenchel orientation efficiency factor. The developed model includes a corrected orientation efficiency factor of cos2(2α), where α is the yarn surface twist angle. The model is validated with extensive experimental data from Goutianos and Peijs (Goutianos S, Peijs T (2003) Adv Compos Lett 12(6):237) and is found to be a near-perfect fit (R2 = 0.960). Experimental data from other studies are also used for further verification. [ABSTRACT FROM AUTHOR]