This study focuses on the design of a horizontal type photobioreactor using internal LED Illumination for cultivation of microalgae such as Spirulina platensis. Photobioreactors with the capacity of 1 and 10 liters are designed with consideration of luminous intensity and flow rate and so on. The capacity of the proposed photobioreactor can be easily changed by replacing only inner and outer cylinder with additional LEDs. Moreover the proposed design can be extended the modular photobioractor system by connecting several photobioreactors. Multilayered wall is proposed to maximize the surface to volume ratio which improves the light efficiency. Moreover, several types of agitators are designed to improve the efficiency of mass transfer for microalgae cultivation. The design parameters of the horizontal type photobioreactor are optimized for the higher productivity of microalgae based on the experiments.