The invention relates to an arrangement in a boom system (1). This type of boom system, specifically in a forest machine, comprises at least two interlinked boom parts (2, 4) and at least one power transmission means (7, 8, 9) connected to the boom parts for moving the boom parts in relation to each other and the environment. In addition, the boom part comprises an elongated body (11) with a hollow structure and at least one set of joint means (5, 6) intersected by turning axles for attaching the boom part to an adjacent structure, and at least one set of attachment means (14) for attaching an actuator controlling the boom system to the body, whereby at least one boom part is formed as a uniform, hollow piece made of fluid material. The cross-section of the body (11) is made to cinch toward the midpoint of the body.
