As interest in pets increases, perception that walking is an important factor for pets has begun. Pets smell various scents while taking a walk, develop sociality and relieve stress through olfactory experiences. Many people take walks before and after work, making walking with their pet a part of their daily lives. Small and hairy pets during night walks are at increased risk of accidents caused by cars, bicycles, and quickboards running from dark alleys or parks. Therefore, this study aims to develop an LED harness that automatically lights up according to the brightness of the light without the pet owner pressing the button during a night walk. Focusing on the theoretical background, previous studies on the development of pet-related wearable computer design were examined, and the importance and concept of walking were summarized. We also looked at harness of various designs sold in Korea. Among them, the vest harness most suitable for small pets were selected and the harness types, materials, colors, and sizes of four domestic brands were analyzed. Based on the results of the theoretical background, the design concept of the LED harness was inspired by One Mile Wear among the 2021 trends, and the material and details of the harness were referenced for the Athleisure Look, which expresses One Mile Wear well. In addition, a prototype of the LED harness was produced using Arduino Lilypad, illuminance sensor, and LED, and a hands-free lead string that can be used with the LED harness was also developed. In fact, clothes were evaluated for a pet and a safe LED harness design was presented for a night walk. Through this, it will be used as a basic data for the development of pet wearable computer design, and will help follow-up research that combines wearable computer technology with clothes and various products for pets as well as harness.