Density is a fundamental physical property of a reinforcement in composite materials. It appears widely in calculations, which are mostly used for engineering designs. The purpose of this research project is to establish a standard test method to measure the density of flax fiber using Archimedes (buoyancy) method. Different immersion fluids were considered in the density measurement of flax fiber with and without use of a vacuum oven. Results of gas pycnometry tests of the same flax sample were used as the reference for comparison. Variables such as, specimen size, vacuum pressure, vacuum time and type of immersion fluid were studied by the ruggedness test. Results of this study suggest that use of certified lab grade soybean oil as test fluid, and submerging flax specimens in test fluid under vacuum pressure of 90–100kPa for 10min yields the closest values of density to those acquired by gas pycnometry method.