Class D amplifiers are routinely employed as audio amplifiers due to their high power efficiency. Total Harmonic Distortion (THD) is one of the most important parameters to qualify and quantify their performance. In this paper, THD of a commonly used Bridge-Tied-Load (BTL) Class D amplifier is investigated, including the derivation of the analytical expression for the THD of the BTL Class D amplifier. We show that in some cases, the THD of the BTL Class D amplifier is independent of the integrator gain of the amplifier - this is unlike other Class D amplifiers and linear amplifiers whose THD is largely determined by their integrator gain. Instead, the THD of the BTL Class D amplifier is largely determined by the feedback amplifier. Further, unlike the single-ended Class D amplifier whose THD increases as the modulation index increases, the THD of the BTL Class D amplifier is maximum at the modulation index = 0.5. The analysis herein provide good insight to the design of BTL Class D amplifiers, including how various parameters may be varied/optimized to meet a given THD specification.