Based on the combination of two approximate theories,effective medium theory and scalar diffractive theory,a novel procedure for designing diffractive lens by use of subwavelength structures is discussed.By comparing the phase shift of continuous relief to that of subwavelength structures,the binary profiles of subwavelength lens,which offer high diffractive efficiencies yet require only single step fabrication,could be determined on the basis of the closed-form expression from effective medium theory.