Relative humidity of ambient air is a critical parameter for crop production as it influences the water balance and photosynthesis process in the plants. A comprehensive review on leafy, fruiting, flowering plants and grains were conducted to determine the effects of varying humidity levels on plant growth. Data from published works concluded that the plant growth improved with increasing humidity, as higher humidity conditions help to keep the stomata open to maintain the photosynthesis process and minimize evaporation process of the plants. By using regression analysis, it is forecasted that the increase in the air relative humidity by 10% would increase the leaf length, width and aerial dry weight for Lactuca Sativa by 10±3%, 2±5% and 28±3% respectively. The data trend suggests that the optimal relative humidity that could improve plant growth is 85±2%, as low humidity below optimum level will increase stomatal resistance, which leads to a reduction of carbon dioxide uptake and photosynthesis rate. Inversely, relative humidity above the optimal levels will adversely reduce the transpiration rate, which negatively affects the plant growth and leaf development due to impaired nutrient assimilation. The outcome of this study provides an insight for the development of plant growth forecasting model.