The present study investigated the effects of elevated carbon dioxide concentration (CO 2 ) and air temperature on the germination of seed bulbils and the seedling vigour of two Chinese yam lines. Plants were grown under two CO 2 levels, ambient and elevated (ambient + 200 μmol mol −1 ), and two mean air temperature regimes, 22.2 °C (ambient + 1.4 °C) and 25.6 °C (ambient + 5.2 °C). Elevated CO 2 did not affect bulbil germination under both air temperature regimes. During the early growth stage, the dry weight (DW) of leaves, vines, shoots, roots, belowground parts (roots + tubers) and whole plants were higher under elevated CO 2 than ambient CO 2 for both lines under the low- and high-temperature regimes. The values of vigour indexes (index I = germination % × seedling length and index II = germination % × seedling DW) were also higher under elevated CO 2 than ambient CO 2 for both lines. These results indicated that Chinese yam seedlings respond positively to elevated CO 2 during the early growth stage. The above:belowground DW ratios were lower under elevated CO 2 than ambient CO 2 in seedlings with very small new tubers for both yam lines, indicating that elevated CO 2 strongly affected the root growth in the early growth stage. The DWs of post-treatment seed bulbils were higher in the elevated CO 2 under both air temperature regimes. The results showed that Chinese yam used a smaller amount of the reserves in seed bulbils under elevated CO 2 than under ambient CO 2 .