Boron-lined ionization chambers (BLICs) have gradually become one of radiation detectors which are used to monitor the neutron/-ray mixed field with large flux around reactors in recent years. In this paper a BLIC with high sensitivity is fabricated and the internal structure is detailed. By using a weak current amplifier with an accuracy of fA, the leakage current of the BLIC is less than 1.0 pA when the high voltage is below 700 V; under the irradiation by the Am-Be neutron source, on the I-V curve of the BLIC there appears a current plateau with a length of 500 V and a slope of 3.7210-4 V-1; when the operating voltage is 400 V, the leakage current of the BLIC is 0.4 pA. Experiments show that neutron signal current measured by the BLIC depends on the relative position of the BLIC with repect to the radiation source; a maximum of 2.0 pA is obtained when the source is 8 cm away from the bottom of the hole in the paraffin moderator. Under the exposure by 137Cs and 90Sr,-ray signal current measured by the BLIC is 1.02.0 pA, but the current plateau is not obvious in-ray field. The neutron sensitivity of the BLIC reaches a level of 1.010-15 Acm2s and the-ray sensitivity reaches a level of 9.010-22 Acm2seV-1. The BLIC shows small leakage current, high sensitivity, and good plateau characteristics, which can be used to monitor the neutron/-ray mixed field around reactors.