Microalgae contain many valuable biological compounds, so their commercial production is of great importance. One of the important challenges in microalga production is their high cost associated with their harvesting processes. Therefore, finding efficient and cost-effective technologies for algae biomass separation is essential for achieving an operational process. The study was conducted to find the influence of pH variation from 4 to 11 on harvesting efficiency and fatty acids component of marine microalgae Thalassiosira sp under laboratory condition. The NaOH and HCl were added to adjust pH value in each treatment. Data was presented as mean ± SD. Comparison between treatments was performed using one-way analysis of variance ANOVA. The results showed that with increasing the pH up to 10, flocculation processes (92%) and concentration factor (20) increased significantly. While, lowering the pH from 8.2 to 4 was not effective in isolating algal biomass. In the next stage, the biomass fatty acids content was measured by both alkaline pH induction method and the centrifuge method. Analyzes showed that the percentage of lipid and saturated and unsaturated fatty acids in biomass collected by alkaline pH induction technique was lower than the centrifugation method. In general, the results of the present study showed that clotting in alkaline pH up to 10 can be suggested as a suitable method for harvesting of Thalassiosira sp microalgae, and if the production of polyunsaturated acids is considered, of consuming high amounts of sodium hydroxide should be avoided.