A bias circuit, a memory system, and a method of boosting a voltage level of a first bit line are provided. The bias circuit includes a first current generator, a second current generator, and a bit line bias generator. The first current generator is configured to generate a first replica charging current according to a charging current flowing through a voltage bias transistor. The second current generator is configured to generate a first replica cell current according to a cell current flowing through a common source transistor. The bit line bias generator is coupled to a first page buffer, the first current generator, and the second current generator, and configured to generate a bit line bias voltage, supplied to the first page buffer, according to a comparison of the first replica charging current and the first replica cell current.