‘Siberia’ Oriental hybrid lily plants were grown under conditions of changing day temperature (DT) and night temperature (NT). Plants were grown under three fluctuating temperature conditions (average 25.8°C, 23.4°C, and 19.9°C) in a temperature gradient growth chamber (TGC), and under three DT/NT levels (28/23°C, 24/19°C, and 20/15°C) in constant temperature growth chambers (CGCs). In the CGC experiment, 35 days after planting (at the visible bud stage), the plants grown under low temperature conditions had high fresh weight (FW) of the stem and stem roots. At the flowering stage in both TGC and CGC experiments, the plants grown under low temperature conditions had a longer stem length and higher FW of the stem, flower buds, stem roots, and bulb compared than those of plants grown under high temperature conditions. The plants grown under low temperature conditions had high dry weight (DW) of the total plant, stem, and bulb, and a high DW/FW ratio of the stem and bulb. In both TGC and CGC experiments, the plants grown under the low temperature conditions showed a high relative growth rate (RGR) and net assimilation rate (NAR) from planting to flowering. These data demonstrate that cooler growing temperatures are advantageous for the accumulation of dry matter in ‘Siberia’ lilies. The low growing temperature enhances the accumulation of dry matter in ‘Siberia’ lily plants via the promotion of photosynthesis by leaves and the absorption of nutrients by well-developed stem roots.