Harmonics individually propagate through a power network based on the equivalent harmonic impedance of a given order at a given electrical point. Due to the numerous harmonic sources stochastically generating or causing harmonics, harmonic cancellation phenomenon and uncertainty in the harmonic sources' rate of harmonic generation and harmonic sources' location, a probabilistic methodology is required to analyze the harmonic propagation phenomenon. In this paper, a probabilistic methodology is proposed to comprehensively analyze harmonic propagation in transmission networks containing many power electronic (PE) devices. Defining, tracing and analyzing several variables influential in the harmonic performance of the network, the consequences of the proposed methodology can facilitate the Transmission System Operator (TSO) operation, suggest the most techno-economic harmonic mitigation solution, recognize future concerns in terms of harmonics and provide a detailed harmonic performance of established or even under devotement network.