Monte Carlo simulation is used to study the magnetic properties of magnetic monolayer films with an Ising-like model in which the exchange interaction and dipolar interaction are considered.The system shows an abnormal magnetization with an increasing of the dipolar interaction at low reduced temperatures.The magnetization is divided into two stages.The magnetization becomes difficult with increasing temperature in low external fields.The reverse process occurs in high external fields.This magnetization behavior results in an abnormal phenomenon that the magnetic entropy changes above zero at low temperatures in external fields.An accurate method is introduced to deal with the long-range interaction.