A comprehensive study to evaluate calcium, in terms of gypsum (CaSO₄.2H₂O) by soil dressing application, on enzymatic browning activity of Polyphenol oxidase (PPO) and internal qualities was tested on lettuce var. Grand Rapids under field conditions. A factorial in completely randomized design was arranged with four replications. The results showed that plants-treated with 50 mg kg^-1 gypsum applied at 40 DAP had the maximal fresh weight of 25.83 g plant^-1. The internal qualities of the lettuce at harvest showed that plants treated with 50 mg kg^-1 gypsum had the maximal chlorophyll content (26.80 mg m^-2), while all gypsum concentrations applied in this study, had less content of ascorbic acid than the control plants. Plants-treated with 100 mg kg^-1 gypsum affected to the lowest level of PPO activity at week 3 after transplanting. Furthermore, gypsum application had no effect to biomass, leaf colour, the contents of phenolic and quinone in lettuce at harvesting stage.