Abstract: Reactive oxygen species (ROS), such as hydrogen peroxide, superoxide and hydroxyl radicals, are by-products of biological redox reactions. ROS can denature enzymes and damage important cellular components. Plants develop antioxidant enzymes, such as superoxide dismutase (SOD) and ascorbate peroxidase (APX) to scavenge ROS and detoxify them. The effect of increased cytosolic ascorbate peroxidase (cAPX) on heat and UV-B stress tolerance was studied using transformed tomato (Lycopersicon esculentum cv. Zhongshu No. 5) plants. This research demonstrates, in either laboratory or field tests, the potential to enhance tolerance to heat, UV-B, and sunscald stress by gene transfer. Overexpression of cAPX in transgenic tomato enhanced resistance to heat (40 ?C) and UV-B stress compared to wild-type plants. When leaf disks were placed at 40 ?C for 13 hours, the electrolyte leakage of disks from wild-type were 93%, whereas two tested transgenic lines (A9, A16) exhibited 24% and 52% leakage respectively. When fruits of wild-type and transgenic plants were exposed to UV-B (2.5mW cm^-2) for five days, the extent of browning was 95%, and 33%, and 37%, respectively. In field tests, the detached fruits from field-grown transgenic plants showed more resistance to exposure to direct sunlight than fruits from wild-type plants. APX activity in leaves of cAPX transgenic plants was several folds higher than in leaves of wild-type plants when exposed to heat, UV-B, and drought stresses.