A semiconductor memory device and a method of operating the same are provided. The method of operating the semiconductor memory device includes detecting a first group of changed bits between first and second page data, by comparing the first and second page data, which are read out using first and second test voltages from the memory cells, respectively, detecting a second group of changed bits between the second page data and a third page data, by comparing the second page data with the third page data read out from the memory cells using a third test voltage, comparing the numbers of the first and second groups of changed bits, and determining one of the first to third test voltages as a read voltage according to the comparing of the numbers of the first and second groups of changed bits.