Dinosaurs are known for their large body size. Sauropod dinosaurs (Sauropodomorpha) had an especially large body size; some species reached 30 m long and 50 tons. Many hypotheses have been proposed to explain this phenomenon. In this study we examined this question using the life history theory. We constructed a simple model of life history with the following assumptions: the body size of immature individuals increases following a logistic equation. A higher quality and availability of food plants make the initial growth rate faster and the final saturating size larger. The increase in body size stops once reproduction starts. Fertility increases with adult body size and food-plant quality. Mortality due to predation is mitigated by a larger body size. We calculated the optimal body size at maturity that would maximize the lifetime reproductive success or fitness. The analysis showed that adult body size increased with food-plant quality and availability but decreased with higher mortality due to predators and other factors. This conclusion is consistent with geological studies that suggest a high quality and availability of food plants in the Mesozoic era, efficient air-sac breathing, and the lightweight bones of sauropod dinosaurs, allowing rapid growth of small individuals.Copyright © 2020. Published by Elsevier Ltd.