Abstract: This paper is devoted to researching on the turndown lapel collar with focuses on the factors affecting it,i.e.,the roll angle,the width of lapel,and thickness of the fabric.Mathematic study was done on the differences between the inside lapel collar and the outside lapel collar of the turndown lapel collar to find out the relationship between roll angle and the thickness of fabric. The study showed that the thicker the fabric,the larger the difference of the curves between the outside and inside lapel collars.A quantitative relationship exists between them.