An ultra-low power complementary metal-oxide-semiconductor(CMOS) front-end readout ASIC was developed for a portable digital radiation detector.The ASIC having a charge sensitive amplifier and a semi-Gaussian pulse-shaper was produced using the CSMC 0.5 μm DPDM process.The ENC noise of 363 e at 0 pF with a noise slope of 23 e/pF complies with the stringent low noise requirements.The peaking time was 250 ns at a 100 mV/fC conversion gain(detector capacitance is 20 pF).By operating this front-end readout ASIC in the weak inversion region,the ultra-low power dissipation is only 0.1 mW/channel(3.0 V).Simulations and test results suggest that this design gives lower power consumption than the front-end readout ASICs working in the strong inversion and is appropriate for the portable digital radiation detectors. An ultra-low power complementary metal-oxide-semiconductor (CMOS) front-end readout ASIC was developed for a portable digital radiation detector. The ASIC has a charge sensitive amplifier and a semi-Gaussian pulse-shaper was produced using the CSMC 0.5 μm DPDM process.The ENC noise of 363 e at 0 pF with a noise slope of 23 e / pF complies with the stringent low noise requirements of 250 ns at a 100 mV / fC conversion gain (detector capacitance is 20 pF) .By operating this front-end readout ASIC in the weak inversion region, the ultra-low power dissipation is only 0.1 mW / channel (3.0 V). Simulations and test results suggest that this design gives lower power consumption than the front-end readout ASICs working in the strong inversion and is suitable for the portable digital radiation detectors.