A sensing circuit includes a cell clock generator, a reference clock generator, a counter, a latching signal generator, a latch and a count-to-state conversion circuit. The cell clock generator receives a cell current from a selected memory cell, and converts the cell current into a cell clock signal. The reference clock generator converts a reference current into a reference clock signal. The count receives the cell clock signal, and generates a count value. When a pulse number of the reference clock signal reaches a predetermined count value, the latching signal generator activates a latching signal. When the latching signal is activated, the latch issues a latched count value. The count-to-state conversion circuit receives the latched count value, and issues a state value. A storage state of the selected memory cell is determined according to the state value.