Oil palm fruits has thick flesh and is rich in oil, and the oil content increases according to the maturity of the fruit. Many methods are developed to detect the maturity level of oil palm fruits, such as destructive and non-destructive methods. The destructive techniques involve physical contact that disrupts the palm fruits's integrity and significantly diminishes the oil yield obtainable from the palm fruits being analyzed. Meanwhile, non-destructive techniques can involve non-contact features that are either visual or non-visual. This research aims to utilize the Free Space Measurement (FSM) approach as a non-destructive method to determine the characteristics of the maturity level of oil palm fruits. The study will analyze three different levels of maturity: unripe, ripe, and overripe. The results obtained state that unripe fruit has a smaller transmission coefficient compared to ripe fruit. This is due to raw fruit having a low water content. Therefore, the level of transmission of electromagnetic waves is also low. Ripe fruit has a higher transmission coefficient than overripe fruit due to the presence of free fatty acids in the overripe fruit, which negatively impacts the oil's quality. © 2023 IEEE.