Abstract: This article analyzed the penetrating and coating of the size on polyester/cotton and pure cotton warps,and the microscopic study of the cross-section,examination of strength enhancement and elongation reduction rates and desizing time of the sized yarns revealed that it is easy for polyester/cotton(65/35) warps to form coating effect in sizing,while the pure cotton warps tend to form penetration sizing effect.The basic reasons for this difference,as shown by analysis,lie in the hydroscopic property of the textile material.Furthermore,the sizing principle and practical applications of both polyester/cotton and pure cotton warps are researched.