Phaeodactylum tricornutum is the microalgae that is known to produce lipid. In this study, it was aimed to estimate the microalgae Phaeodactylum tricornutum dry matter cultured in the conditions of nitrogen deficiency as a stress factor, in outdoor, in photo bioreactors, by using multiple regression analysis method. In this study, diatom Phaeodactylum tricornutum (Bohlin) was cultured medium of which N was reduced by 50% and with 20% inoculation ratio. The light intensity, temperature, optical density and chlorophyll a, were measured daily. The mathematical model was formed for control group and 50 % of the N applied to the group using optical density, temperature, light intensity, chlorophyll a and without chlorophyll a. The regression equation of control group was estimated. The R2 value of control group was found 95.1% and statistically significant. Then, the regression equation was estimated for control group without chlorophyll a. The R2 value of this equation was found 94.0% and statistically significant. And then, the regression equation was estimated in 50% nitrogen. The R2 value of control group was found 92.4% and statistically significant. Finally, the regression equation was estimated in 50% nitrogen deficiency without chlorophyll a. The R2 value of this equation was found 91.7% and statistically significant.