In this paper, a design optimization method for left-handed materials with specific frequency bands is proposed. A microstructure of the left-handed material is constructed based on the transmission line theory, and the properties of this material are simulated numerically with the S-parameter retrieval method and analyzed with equivalent transmission line model. The effects of the size parameters of the microstructure on the left-handed characters are analyzed. The analysis results verify that this structure does exhibit left-handed characteristics with appropriately sized microstructure. Then the mathematical model of the microstructure with maximized frequency band-width and lower loss at given frequencies is proposed, and examples are given. The design result confirms that the optimized model is applicable and accurate.