Abstract: Formulas for detection efficiency and sensitivity of plate boron-lined ionization chamber were derived from its working principle, and their values for thermal neutrons were obtained. The saturated value of the detection efficiency for thermal neutrons of ionization chamber is 1.35% and the sensitivity is 9.65×10^-14 A•cm^-2•s^-1, which approximately coincides with the result of 8.43×10^-14 A•cm^-2•s^-1 given by a known formula. Contribution of α particles to the detection efficiency shows small difference with Li ions while that to the sensitivity is predominating. A suitable thickness of boron film, slowing fast neutron and using enriched boron can improve both detection efficiency and sensitivity of boron-lined ionization chamber.