In this paper, a nanowatt comparator for always-on microelectromechanical systems (MEMS) switch wake-up sensor is presented. To achieve continuous event detection with nanowatt ultralow power (ULP) consumption, the comparator building blocks are designed to work in subthreshold region. Various circuit techniques including self-cascode transconductance, feedforward slew rate enhancement are adopted to optimize the circuit performance under nA power consumption. Programmable comparator hysteresis allows the flexibility to configure the comparator to operate in different environments. PVT-insensitive voltage/current bias circuits are included to provide stable bias to ensure robust operation across the process corners. The proposed circuit is designed and implemented in a standard 1P8M 0.13-µm CMOS process. The core circuit area is 220µm × 60µm. Post layout simulations show that the overall comparator power consumption is in the range of 5.2 − 7.8nW under 0.8V − 1.2V supply, the corresponding work bandwidth is up to 6kHz.