Harmonic resonances exist inherently in power systems with varying affected areas and amplification levels. This paper presents a quantitative harmonic resonance assessment technique with sensitivity analysis under uncertainty to reveal the affected areas of a harmonic resonance and corresponding amplification severities at different buses. Resonance amplification conditions at the harmonic source bus (localized resonance) and non-harmonic-source bus (non-localized resonance) are distinguished, respectively, and then a novel index to quantify the harmonic propagation areas and corresponding amplification severities are established based on harmonic electrical distances. Apart from local sensitivity analysis (SA), a global SA method is further used to perform the SA of system uncertain parameters, and three types of sensitivity parameters are distinguished and defined. In addition, comparative analysis of the proposed method with existing harmonic resonance analysis methods shows that the proposed method can not only provide more information for harmonic resonance analysis and control but is also easy and practical.