Abstract: In order to accumulate the basic physiological index data for juvenile yellowfin tuna (Thunnus albacores), we evaluated the blood potassium ion (K⁺), calcium ion (Ca²⁺), chloride ion (Cl⁻), hydrogen ion (H⁺), pH, glucose, lactic acid, sodium ion (Na⁺) and total hemoglobin concentration and hematocrit, osmotic pressure of the juveniles of different body lengths (20G: 20−30 cm, 30G: 30−40 cm, 40G: 40−50 cm). Results indicate that the blood parameters of the juveniles were relatively stable in the three body length groups. K⁺, Ca²⁺, Cl⁻, H⁺, pH, glucose and hematocrit were not significantly different between groups (P>0.05). The content of lactic acid and total hemoglobin concentration in the 20G group was significantly higher than the other two groups (P<0.05). The standardized calcium ion in the 30G group was significantly higher than the other two groups (P<0.05). The sodium ion content decreased with the increase of body length gradually, and the difference between the adjacent two groups was not significant (P>0.05). The results show that the juveniles had a small difference between the 30G and 40G groups. The 20G group was relatively different from the other two groups. As body length increased, blood indicators tended to be more stable.