This study uses the Experimental and Computer aid methods to analysis the defrost effect of windshield under various seasons which with rainy days or high humidity. The thermal resistance concept is used to find out the inner and outer temperatures of the windshield of a vehicle. Then applying the inner and outer air temperatures as well as relative humidity to find out the dew points of the air next to the windshield from experimental formula or psychrometric chart. Using 12 different windshield defrost methods: including only open cooling air or heating air or open the cooling and heating air simultaneously; to blow towards windshield or towards passengers; using inside air or outside air. By comparing the temperatures of the windshield and the dew points of the air next to windshield to analysis the defrost effects of windshield. The windshield defrost experimental results are match the numerical results.