One of the most challenging issues when operating a high-power light-emitting diode (LED) is to conduct appropriate packaging materials for the reliable thermal management of the entire device. Generally, the considerable amount of heat produced around the junction area of the LED would transfers to the entire device, and causes thermal expansion in the packaging material. It induces strain inevitably, and hinders the output performances and possible applications of high-power LEDs. The coefficient of thermal expansion (CTE) is a physical quantity that indicates the expansion to which value a material will be upon heating. Therefore, as far as an advancement of thermal management is concerned, the quantitative and real-time determination of CTE of packaging materials becomes more important than ever since the demanding of high-power LEDs is increased in recent years. In this study, we measure the CTE of GaN-based (λ = 450 nm) high-power LED encapsulated with polystyrene resin by using optical coherent tomography (OCT). The displacement change between individual junctions of OCT image is directly observed and recorded to derive the CTE values of composed components of the LED device. The obtained instant CTE of polystyrene resin is estimated to be around 10 × 10−5/°C, which is a spatial average value over the OCT scanning area of 10 μm × 10 μm. The OCT provides an alternative way to determine a real-time, non-destructive, and spatially resolved CTE values of the LED device, and that shows essential advantage over the typical CTE measurement techniques.