Ring vibratory gyroscopes are important angular rate sensors widely used in inertial navigation systems. A highly symmetrical resonator is the core part of the ring vibratory gyroscope. Frequency split is a key parameter which denotes the level of unbalanced mass and stiffness of the resonator. Many research works focus on the precise machining of the resonator for the sake of eliminating frequency split. However, for metallic ring resonators, the decrease of frequency split is not always proportional to the promotion of geometric accuracy. This paper investigates the frequency split of the ring resonator gyroscope caused by parametric errors including geometric and material imperfection via a method of harmonic transformation. The roundness error of the ring resonator is extracted, and then decomposed to a series of orders of harmonic waves. Transformation results show that for the tested resonator, its first 20 orders of harmonic waves contain the main components of the roundness error. Then a precise FEM modeling is built for frequency split analysis. The simulation result shows that the roundness error of the resonator can cause a frequency split of 0.6 Hz, which accounts for 16.2 % of the total frequency split. Based on the metallographic observation and grouping experiment of different metallic resonators, it is deduced that the main frequency split is caused by material heterogeneity. It shows that the material homogenization is as important as precise machining for the resonator of small frequency split. The proposed research provides an instruction to manufacture high quality metallic resonators.