The paper describes the investigations of the authors on the methods of microfabrication to obtain important silicon MEMS (microelectromechanical systems) devices met in applications of microsensors and microactuators from micromechanics and microrobotics. There are considered an acceleration piezoresistive microsensor performed by bulk micromachining technique using crystalline anisotropy property of the material, monocrystalline silicon wafer, and as well as some thermal microtweezers made from silicon polycrystalline by surface micromachining using sacrificial layer technique. Simulations of modal analysis, and thermal and static structural analysis of the investigated structures are also performed. We comment on the reliability aspects from the perspective of the performance of the devices under study, imposed by the applications and influenced by the material, the fabrication method, the dimensions of the structures, the modulus of elasticity, the fundamental frequency, the vibration modes, and the thermo-mechanical characteristics, presented through simulations.