The ISO 2370 standard specifies a test method for determining the fineness of flax fibres using the osmotic method. This method indirectly reflects the fineness of flax fibres by evaluating their permeability characteristics in liquids. This test method is simple and efficient, and can provide consistent and reliable fineness data for the Spinning & weaving industry, thus helping to optimize the selection and processing of flax fibres. The osmotic method is particularly suitable for quality control and standardized evaluation of high-volume fiber samples, helping to ensure that the performance of fibres meets the needs of production and use. The standard is widely used in Spinning & weaving, fiber processing and other industries to provide a scientific basis for mass management and product performance optimization of Spinning & weaving products.