Contradictory results on the effects of day and night temperatures on plant growth and development have been reported in the literature. This is partly due to the lack of unified approach to data analysis and interpretation of results. In the present paper a method for quantifying the effect of day and night temperatures, their difference (DIF) and average daily temperature (ADT) on plant growth and development is described. The proposed method is based on the model calculations and subsequent analysis of isopleth plots developed from the equations. It enables an easier and more precise analysis of the plant response to varying day and night temperatures in terms of their absolute values and their difference. The method was verified on cucumber, lily, petunia and sweet pepper.