The unvarying condition diesel engines used for commercial applications, transportation and industries lead to the crisis of petroleum fuel diminution and ecological squalor caused due to exhaust gases. Therefore, in this paper optimization of the use of MSB in naturally aspirated, direct injection diesel engines, parameters of pure diesel (D100), 80% diesel + 20% microalgae spirulina (B20), 60% diesel + 40% microalgae spirulina (B40) and pure microalgae spirulina biodiesel (B100) were investigated at various fuel injection pressures (FIP) of 18 to 26 MPa and stationary injection timings (23.5° b TDC). The result shows that optimum effect can be obtained in 22 MPa FIP, with B20 bio-diesel without compromising the performance against diesel. B20 blend presented less NOX and smoke emissions by 13.7% and 22.2% respectively with no significant change in engine performance when compared to diesel at full load operating condition. The simulation and experiment results are verified at the same operating conditions.