A large number of high voltage substations in Thailand transmission network is now reaching end of lifetime. To renovate these aged substations, it requires a high capital investment from the utility. Moreover, the detail of renovation task is complicated due to the previous uprating, upgrading or renovating of some bays or some equipment in substation to fulfil the system requirement such as the increasing of load current or short circuit current [1]. Therefore, the requirement and detail of work to renovate these aged substations must be comprehensively accessed to determine whether the whole substation or only some bays or some equipment in a bay should be renovated [2]. Thus, this paper proposes a performance assessment of aged high voltage substations in Thailand transmission network. The performance assessment procedure has been firstly setup based on technical and economical aspects of aged high voltage equipment such as power circuit breaker, disconnecting switch, instrument transformer, and surge' arrester. To evaluate the equipment performance, six criteria have been defined consisting of age, electrical stress, symptom or sign of failure, failed type, obsolescence as well as environmental and safety impacts. The equipment performance evaluation is performed by applying the Weighted Scoring Method (WSM) and used the multi-criteria decision making process known as the Analytical Hierarchy Process (AHP) to determine the weighting of each criterion. Subsequently, the known performance of individual high voltage equipment in each substation bay is combined together to define the bay performance known as Bay Performance Index (%BPI). The known performance of each bay is further used to calculate the substation performance known as Substation Performance Index (%SPI). This known bay and substation performance index is used to prioritize the renovation tasks, meaning that the substation or bay having low performance index should be renovated first. The setup procedure has been further developed as a decision support tool for substation renovation planning in the web application software with central database of all high voltage equipment. After the software was completed, the performance of 54 aged high voltage substations has been accessed by using the developed web application software with satisfactory result. The result of equipment, bay and substation performance has been designed to display according to traffic light colour as red for poor, yellow for moderate and green for good performance, respectively. Finally, with this developed software, the performance of all major equipment, bay and substation in Thailand transmission network can be accessed for renovation planning, resource allocation as well as spare equipment management.