An ultra-low power area-efficient fully differential multi-bit quantizer used in Sigma-Delta modulator (SDM) is presented. The quantizer is named as capacitor-string area-efficient (CSAE) quantizer since instead of the typical resistor string, it employs a capacitor string to avoid static power consumption. Furthermore, a novel circuit configuration with saving half number of comparators is applied to it to save area for cutting cost of chip. The novel multi-bit quantizer is designed in a standard 0.35μm CMOS process. Simulation results show that a 17-level CSAE quantizer clocked at 32 kHz achieves a power consumption of 0.225μW when the power supply voltage is 1.8V. Its area is 0.145mm2. Compared with traditional design, the proposed CSAE quantizer saves 81% power and 37% area and thus is suitable for ultra-low power applications.