Density is a fundamental physical property of a reinforcement for composites. It appears widely in calculations, which are mostly used for engineering designs. The density of homogeneous solid materials is generally straightforward to determine, because it requires only measurements of mass (or weight) and of volume of a specimen. Flax fiber requires a different method of density measurement because it is a porous material and possesses unique surface chemistry. Two methods can be utilized to verify the density of natural fibers, but they have their particularities. The first one is called Gas Pycnometry, which is more precise. The second is called Buoyancy (Archimedes), and it is acceptable as an alternative to the first cited in applications where less precise results are sufficient. The second method was chosen for the experiment with flax fiber. One of the reasons for this choice was the lower cost of the equipment, since it uses a simple density measurement kit for laboratory mass balances. For the Pycnometry test, an expensive machine must be acquired and maintained. The main issues of the experiments were to verify which fluid and vacuum chamber pressure prior to immersion could give the best results. Types of oil used for the experiments include: canola, soybean and mineral oil. A range of pressure used was between 50-100kPa. It has been realized from the experiments that the vacuum chamber pressure is more significant than the type of fluid, but only when high pressure is used.