An important trait of Pacific bluefin tuna (PBT) is its ability to maintain body temperature through high metabolic heat production and its retention ability. PBT rapidly develop the thermo-conservation ability between 20 and 40 cm fork length (FL), which has been attributed to improved insulation. Meanwhile, how heat production capacity develops during ontogeny is not well understood. The aim of the present study was to explore the ontogenetic change in metabolic heat and its contribution to endothermic ability through swim-tunnel respirometry and a heat-budget model. Swim-tunnel respirometry was conducted on PBT, ranging from 19.4 to 27.5 cm FL (94–330 g, n=16) in Kochi Prefecture, Japan, from August to September 2022 and 2023. The metabolic rate of each PBT was calculated as the standard metabolic rate (BSMR) at a swim speed of 0 cm/s and the minimum metabolic rate (BUmin) at a minimum swim speed of 44.3 cm/s. The scaling exponents of BSMR and BUmin were estimated at 1.03 and 0.93, respectively. Both scaling exponents of metabolic rates were higher than those estimated for the tribe Thunnini above 300 g. Furthermore, a scaling exponent close to 1 implies that heat production rate (Tm in °C/min) in heat-budget model does not decrease in this body size range. To explore the ontogenetic change of heat-production rate of PBT in nature, heat-budget model was applied to bio-logging data collected from nine juveniles (size of analysis: 23–60 cm FL; 200–5272 g) with electronic archival tags inserted into their body cavity. The heat-production rate of each PBT did not decrease until approximately 800 g and declined thereafter, supporting the results of swim-tunnel respirometry. Furthermore, we evaluated the effects of heat production rate on the thermo-conservation ability, the difference between body temperature and water temperature, and found that maintaining a high heat production rate mainly contributed to the initial stage of temperature difference generation. Our results provided new insights into the endothermal development of tuna.