The fourth passive circuit element, memristor, has attracted increased attentions since the first real device was discovered by HP Lab in 2008. Its distinctive characteristic to record the historic profile of the voltage/current through itself creates great potentials in future system design. However, as a nano-scale device, memristor is facing great challenge on process variation control in the manufacturing. In this work, we analyze the impact of the geometry variations on the electrical properties of both TiO₂ thin-film and spintronic memristors, including line edge roughness and thickness fluctuation. A simple algorithm was proposed to generate a large volume of geometry variation-aware three-dimensional device structures for Monte-Carlo simulations. Our simulation results show that due to the different physical mechanisms, TiO₂ thin-film memristor and spintronic memristor demonstrate very different electrical characteristics even when exposing them to the same excitations and under the same process variation conditions.