This research proposes technical and financial evaluation for investment of harmonic mitigation in power network. Main proposal of research is analysis of harmonic impact and harmonic solution by regarding to economic decency for investment. The parallel resonance is harmonic impact that will be mentioned in this research because it is a problem that is mostly emerged in industries that contain non-linear loads and power factor correction devices. Its impact on power network comprises high voltage and current amplification, which linked to equipment and system failure. An application of detuned filter will be proposed to solve the problem due to its popularity (It is a basic solution, easier and more inexpensive than the other solution). The parallel resonance condition will be modeled with an industrial power network, which contains a power transformer, a linear load (AC motor), a non-linear load (AC variable speed drive; AC VSD), a power factor correction device (capacitor bank) and a harmonic correction device (detuned filter bank), by using electrical, mathematical and payback period formulas. The results will illustrate the impact of the parallel resonance phenomenon in power network due to harmonic distortion, problem mitigation by using a detuned filter bank and comparison of payback period between power factor correction by using a capacitor bank and both of power factor and harmonic correction by using a detuned filter bank. Above evaluation results can be applied to information for investment of power factor correction and harmonic mitigation in the actual power network.