This paper reports a wafer-level package (WLP) encapsulation and a stacked multi-chip package (MCP) for electrostatically actuated MEMS variable capacitor and control IC chips. Since the MEMS capacitor needs to be operated in dry atmosphere, we developed the WLP encapsulation structure with four thin-film layers fabricated by conventional back-end-of-the-line (BEOL) LSI technologies. Furthermore, since a MEMS chip is actuated by a control IC chip that provides high voltage, we developed the stacked MCP to integrate the MEMS chip and the control IC chip. In order to handle relatively fragile WLP encapsulation, the stacked MCP process was optimized. The stacked MCP has 4.5×4.5mm size and 0.8mm thickness, which is the thinnest package for stacked MEMS and IC chips. Normal operation of the MEMS variable capacitor in the stacked MCP confirmed up to 85%RH after reliability tests, i.e. temperature cycling, accelerated moisture resistance, and moisture/reflow sensitivity.