Abstract: An aging status diagnostic apparatus for a power conversion system and diagnosing method thereof are provided. The apparatus includes an output current sensing means detecting output current of an inverter switching module; and a measurement and diagnosis means receiving the output current, calculating one or more average values of the output current over one period, and magnitude or effective value of each harmonic of the output current, and determining whether aging of the inverter switching module has occurred. Furthermore, the measurement and diagnosis means determines that the aging of the inverter switching module has occurred if the average value of the output current over one period increases by a value equal to or greater than a predetermined range and/or an even order harmonic or a specific order harmonic based on FFT (Fast Fourier Transform) of the output current increases by a value equal to or greater than a predetermined range.