Abstract: A device for smoothening the unwinding from the cylinder of the loops of yarn of the weft reserve in weft suppliers for fluid jet looms, is described. The device comprises a pneumatic device capable of subjecting the yarn to a fluid flow, such as an air stream, having at least one motion component directed in the running direction of the yarn. The flow, independently from the dimensional and superficial characteristics of the yarn, maintains the latter constantly under mechanical tension, so that the slackening of the loops wound on the terminal portion of the cylinder of the supplier is prevented.