Abstract: By using WinRhizo 2005a root scanning measurement system and traditional paraffin sectioning method, this paper studied the root morphology and leaf structure of tomato seedlings under effects of sub-low temperature (8-15 ℃) and mild drought stress (55%-70% of normal field capacity). Single treatment of sub-low temperature or mild drought stress promoted the increase of the seedling root length, surface area, and volume, while the interaction of the sub-low temperature and mild drought stress resulted in the decline of the indices. At sub-low temperature or under mild drought stress, the root length increase was promoted via the increase of the proportion of 0.50-1.00 mm diameter class roots, and the increase of the root surface area and volume was promoted via the increase of the proportion of 1.00-2.00 mm diameter class roots. Under the interaction of the sub-low temperature and mild drought stress, the root growth was inhibited owing to the decrease of the proportions of 0.20-0.50 mm and <0.20 mm diameter classes roots. There was a significant negative correlation between the cell tense ratio (CTR) and spongy ratio (SR), and a significant positive correlation between the CTR and palisade tissue thickness/spongy tissue thickness. At sub-low temperature, the palisade tissue thickness decreased; under mild drought stress, both the palisade tissue thickness and the spongy tissue thickness decreased; whereas the interaction of sub-low temperature and mild drought stress made leaf become thinner and palisade tissue thickness/spongy tissue thickness had a significant decrease.