Since structural capacity of RC members depends strongly on localized condition of reinforcements, it is important to model the spatial variability of steel corrosion. However, steel corrosion in RC members can only be observed after severely damaging the concrete member. In order to understand the growth process of steel corrosion and how the spatial variability of steel corrosion increases with time a continuous monitoring is necessary. In this study, X-ray photography is applied to observation of steel corrosion in RC members. The steel weight loss is estimated by using a digital image processing of X-ray photogram. Comparing the steel weight loss estimated by weight measurement with that estimated by the digital image processing, applicability of X-ray photogram for the observation of corrosion process in RC members is demonstrated.