For measuring and/or monitoring properties of yarns or ropes with the aid of an image sensor a two-dimensional image of a portion of the yarn or rope is acquired and converted to electrical image signals. The image signals are digitized in image point manner and stored in an image signal memory at memory locations assigned to the image points. By a calculating unit the values of the properties to be detected are determined from the stored digital image signals. The properties which can be measured and/or monitored in this manner are in particular the diameter of the yarn or rope, the statistical values of projecting fibres, filaments or wires and the twist of the yarn or rope.