This study was divided into two parts. In the 1st trial, we adjusted the levels of protein included in the diets fed to Girella (Girella mezina), then we collected body length and body weight data to determine the level of protein with the best growth performance. In the 2nd trial, which was based on the results of the 1st trial, we used Spirulina powder to replace the fishmeal used in the diets, and then collected body length and body weight data to determine the optimal dosage of spirulina powder. In the 1st trial, G. mezina juveniles with a mean initial length of 4.16±0.72cm and mean initial weight of 6.51±0.53g were fed one of four diets with different dietary protein levels of 35, 40, 45 and 50% for 6 weeks, and it was found that the growth performances of the CP40, CP45 and CP50 groups were significantly better than that of the CP30 group. The weight gains of the groups were 79.90±4.96%, 97.97±7.87%, 100.20±6.71% and 101.88±5.70%, respectively, and the specific growth rates were 1.40±0.01%, 1.63±0.02%, 1.65±0.01% and 1.67±0.03%, respectively. Considering the costs of the diets and the growth performance results, this study suggests that the best dietary protein level is 40%. In the 2nd trial, juveniles with a mean initial length of 4.18±0.79cm and mean initial weight of 6.61±0.56 g were fed on the experimental diets for 6 weeks, and it was found that the growth performances of the SP0, SP25 and SP50 groups were significantly better than those of the other groups. The weight gains were 98.70±6.26%, 99.68±2.41%, 97.52±1.29%, 79.36±6.36% and 78.35±5.75%, respectively, and the specific growth rates were 1.63±0.01%, 1.65±0.02%, 1.62±0.02%, 1.39±0.02% and 1.38±0.01%, respectively. According to the results of the statistical analysis, the optimal dosage of spirulina powder is 50%.