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This paper describes an analysis and a performance limit of a vibrational energy harvester with a novel slit-and-slider structure. This structure has a separable electret and microelectromechanical systems (MEMS) parts. In the MEMS parts, movable electrodes slide due to external vibration and receive electrical field that is periodically modulated by slits of fixed electrodes. The structure was fabricated based on MEMS technology and produced an ac current of 170 pA with an external vibration of amplitude of 1 m/s2 at a frequency of 1166 Hz. Since the structure is separable, individual characterization of the electret and movable electrodes was performed. On the basis of their quantitative analyses, a structural model was constructed and validated. The model showed a way to optimize structural and material parameters for enhancement of output power and predicted a performance limit of 2.5 × 10-3 μW and 6.1% as output power and harvester effectiveness, respectively. This value of effectiveness is comparable to that of conventional non-MEMS-based large energy harvester around 1 cm3, which indicates feasibility of MEMS-based small energy harvesters around 0.01 cm3 by appropriate designing.