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Fiber-reinforced polymers (FRPs), which are widely used in ship structures, have numerous advantages. However, environmental concerns exist regarding the raw materials’ characteristics and challenges associated with disposal. Natural fibers such as hemp and flax are promising alternatives already applied in the automotive industry; nonetheless, their application to marine structures remains limited. Fiber content ( Gc ) measurements are crucial to satisfy structural design regulations, for which several methods exist. Among others, the burn-off method recommended by ISO 12215-5 and classification societies can accurately measure the Gc and internal defects of fiber composites. However, natural fibers have low ignition points, which renders the conventional burn-off method unsuitable. This study analyzed existing measurement techniques designed to assess Gc and internal defects considering the unique characteristics of cellulose-based natural FRPs (NFRPs). A modified burn-off method for NFRPs, which incorporates a pre-heating stage and adjusted combustion temperatures, is proposed to address the high moisture absorption and ignition point that are lower than those of resins inherent to natural fibers, overcoming the limitations of standard burn-off procedures. Experimental hemp and flax fiber composites were fabricated and their Gc was measured using various conventional methods and two proposed techniques: pre-heating followed by hydrometer testing and the modified burn-off method. Accordingly, the hemp specimens exhibited an average moisture absorption of approximately 2.623 ± 0.976 wt%, whereas that of the flax specimens was approximately 1.877 ± 1.115 wt%, and both were found to contain higher levels of internal defects compared to conventional glass-fiber-reinforced polymers. Overall, the modified burn-off method enables accurate quantitative evaluation of both Gc and internal defects in NFRPs. • A method for measuring the Gc and internal defects of NFRPs is proposed. • The modified burn-off method has a pre-heating step and adjusted burn temperature. • More precise measurement of Gc and inner defects is achieved. • The method is validated through comparison with standard measurement methods. • Accurate Gc measurement is crucial for NFRP application in ship structures.