Abstract: The inductances of the primary and second coil are important parameters of Tesla transformer. Some elementary research is carried out on the magnetic circuit of Tesla transformer with open circuit magnetic core, including the calculation on magnetic force lines and the magnetic flux density distribution all over the Tesla transformer magnetic circuit. The result shows that, in the magnetic core regions, the magnetic force lines mainly concentrate inside magnetic cores, and in the region between inner and outer magnetic cores, the magnetic force lines mainly distribute between the primary/second coil ends and the magnetic core ends. Practically, the magnetic leakage of Tesla transformer (Generally, the length of Tesla transformers primary and secondary coil is half of the length of magnetic core.) is comparatively small. Given the assumption that there is no magnetic leakage, we estimate the primary and secondary coil inductances of Tesla transformer. The error of the estimation is less than 15% compared to the measurement, which indicates that the arithmetic introduced in this paper is a convenient and advantageous method to solve the problem of design and parameters estimation of Tesla transformer.