Propylene is prepared by a process for producing propylene, comprising: contacting a reaction mixture of ethylene and methanol and/or dimethyl ether in the presence of a catalyst while controlling the amount of ethylene that is discharged from the reaction system to a reduced level in comparison to the amount of ethylene that is being fed into the reaction system, and while controlling the yield of propylene to at least 40 mol %, based on the sum of the number of moles of methanol and two times the number of moles of dimethyl ether, which are being fed into the reaction system.