Because of the many advantages that bio-based composites have, their use has increased significantly in recent years. Many experiments have emphasized the good properties of these materials. The objective of this paper is to study the influence of water aging of bio-based flax/PLA composite on their vibration behavior. The material used in this analysis is a polylactic acid (PLA) tape reinforced with flax fiber filament. It's made of a bio-based, biodegradable, and recyclable fiber. The composites that were studied, were produced by 3D printer and immersed in tap water at ambient temperature. First, the water absorption is tested by immersing five samples and checking the mass gain at each period. Secondly, for different percentages of absorption, the vibration behavior of aged materials was investigated using a free vibration test. Next, it was compared with the non-aged composite behavior, which is considered as the reference. When measured in terms of aging period, water absorption causes a decrease in Young’s modulus and an increase in loss factors. Results show that bio-based composite is very sensitive to humidity, that is why, the stiffness is weakened but the loss factor is improved.