Thermal stress in plants is any temperature increase above the threshold level for a period of time which causes immutable damage in plant growth. At very high temperatures associated severe cellular damage and cell death may occur within a few minutes as a result of disassembling the cell structures. Direct damage from heat stress includes disruption of the protein structure and an increase in fluidity of cell membranes, as well as indirect damage. The result of these disorders is damage to the plants that prevents growth. In the current study, the occurrence time of thermal waves or some climatic phenomena in Qazvin Province were studied over a 25-year period (1982-2007) and an assessment was made of the impact of thermal stress on corn plant growth. The results showed that the highest thermal waves were seen in the months of June, July and August in the years 1985, 2000 and 2001. More heat waves occurred during July in the years 1983, 1995, 1997, 2003 and 2006 while in August more heat waves were experienced in 1983, 1983 and 2005. Over this 25-year period, only on 10 days in June, 63 days in July and 50 days in August did the temperature reach above 38 degrees when corn plant is under stress. In these three months, more negative impact was found in July that caused crop quality reduction or even the death of the plant. When thermal waves increased and rose above than plant endurance threshold, this had a negative impact on production and caused the production rate to reduce or, in some cases, collapse completely.