An arrangement in a boom system, specifically in a forest machine, includes at least two interlinked boom parts and at least one power transmission connected to the boom parts for moving the boom parts in relation to each other and the environment. The boom part includes an elongated body with a hollow structure and at least one set of joints intersected by turning axles for attaching the boom part to an adjacent structure, and at least one set of attachments 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 is made to cinch toward the midpoint of the body.