Since it is considered that washing has much effect on wearing out of dresses at the seams, we studied this effect by measuring the breaking strength of seam thread and unused thread. The samples of fabrics were cotton and polyester-rayon and threads were cotton and polyester. The results obtained are as follows : 1. For both cotton and polyester-rayon fabrics, the breaking strength of polyester thread at the seam was little affected by washing. 2. On the other hand, the breaking strength of cotton thread at the seam was significantly lowered by washing. 3. The unused cotton thread was damaged by washing more heavily than polyester thread. 4. In either case of unused thread and seam thread, the coefficient of variation of breaking strength of cotton thread was larger than that of polyester thread.