This paper studies several magnetic design aspects of the trans-rotary magnetic gear (TROMAG). The TROMAG is a magnetic device that converts linear motion to rotation, and vice versa, through magnetic field while doing a gearing action. This means that a high-force, low-speed translation can be converted to a high-speed low-torque rotation. Once coupled with a conventional rotary machine, the resultant system may be used as a compact electromechanical motion system for high-force linear-motion applications, such as wave energy conversion. The paper investigates the magnetic design of the TROMAG by means of either two-dimensional (2-D) or three-dimensional (3-D) finite element analysis (FEA), or an accurate analytical model. The impact of nonideal discretized helix and dimensions of magnets and air gap are investigated, as well as scaling rules of the TROMAG and the possibility of demagnetization. Experimental results obtained from a lab prototype are also presented to verify the concept and analysis.