Abstract: Background and Objective: Chlorophyll a and b are the main pigments require magnesium for light energy and the synthesis of both. Kieserite (MgSO4·H2O) contains magnesium which plays an important role as the central atom of the porphyrin ring of green plant pigments, chlorophyll a and b. The aim of the study was to evaluate the role of magnesium in kieserite on physiological characteristics of soybean varieties. Materials and Methods: The study was conducted in Deli Tua, Deli Serdang Regency, Sumatera Utara from May to August, 2019. The experimental design used was a factorial randomized block design. As the first factor, soybean varieties consist of Demas, Anjasmoro, Devon-1, Dering-1. The second factor is dose of kieserite (MgSO4.H2O) applied consisting of 0, 50, 100 and 150 kg kieserite ha–1. Results: The study result indicated that the highest of chlorophyll content was found in Anjasmoro variety compared to other varieties. The highest total chlorophyll and stomatal density was found in combination of Anjasmoro and 50 kg kieserite ha–1. Conclusion: The treatment 50 kg kieserite ha–1 on Anjasmoro increased the chlorophyll content about 30% and stomatal density 15.56%.