Replacing synthetic fibres with natural ones is key for clean and sustainable manufacturing. Natural fibres require characterization to investigate their usefulness. However, current testing methodologies require a large sample size due to the strong variation of tensile strength (TS) among the fibre of the same plant species. The development of a simpler test method to predict fibre TS would help in the selection of the most appropriate plant species with desirable TS values while performing fewer tests; thus saving on time and cost. FT-IR results that determine fibre crystallinity through specific IR absorbance peak ratios may serve to predict the fibre TS. In this study, we tested the relationship between fibre TS and cellulose crystallinity for seven different species of Pandanus spp. Thus, empirical mathematical relationships between the fibre TS value and indices obtained by three specific IR absorbance peaks ratios, namely the total crystallinity index, lateral order index, and hydrogen bond index have been established with a good correlation coefficient (96, 98, and 95%, respectively).