Long mixing time, high power consumption, and small mass transfer coefficients are common problems in the photobioreactor design for microalgae culture which have a great effect on system efficiency and performance, CO2 stabilization, and biomass production. In this study, a special design of the triangular external loop airlift photobioreactor was studied. The bioreactor's geometry was such that the angle between hypotenuse and the horizontal side ( ) could vary. This configuration created an effective gas-liquid countercurrent flow in the downcomer section. In the present research, hydrodynamic and mass transfer of the reactor were investigated on the microalgae productivity under different design and operating parameters. The optimum conditions for the enhancement of Chlorella vulgaris productivity were explored by analyzing the mixing time ( ), volumetric power consumption (P/V), mass transfer coefficients ( ), bubble diameter (d), and gas holdup ( ) as responses. The results showed that the hypotenuse angle of = 59o and the superficial gas velocities of the = 0.0050 m.s-1 for the downcomer and = 0.008 m.s-1 for the riser of the reactor were the best conditions to achieve the highest biomass productivity. The responses’ values obtained in the optimum condition were as follows: = 19.67 (h-1), = 23.79 (h-1), = 23.76 (h-1), = 0.41, and = 62.83 , which had a smaller deviation than the actual values. The highest concentration of Chlorella vulgaris ( 1.4 g.l-1) achieved in this work was obtained in a shorter span of time (11th day of cultivation) based on the growth curve in optimized conditions.