The potential of Mentha piperita in the iron nanoparticles (FeNPs) production was evaluated for the first time. The influences of the variables such as incubation time, temperature, and volume ratio of the extract to metal ions on the nanoparticle size were investigated using central composite design. The appearance of SPR bands at 284 nm in UV-Vis spectra of the mixtures verified the nanoparticle formation. Incubating the aqueous extract and metal precursor with 1.5 volume ratio at 50°C for 30 min leads to the formation of the smallest nanoparticles with the narrowest size distribution. At the optimal condition, the nanoparticles were found to be within the range of 35-50 nm. Experimental measurements of the average nanoparticle size were fitted well to the polynomial model satisfactory with R2 of 0.9078. Among all model terms, the linear term of temperature, the quadratic terms of temperature, and mixing volume ratio have the significant effects on the nanoparticle average size. FeNPs produced at the optimal condition were characterised by transmission electron microscopy, thermogravimetry analysis (TGA), and Fourier-transform infrared spectroscopy. The observed weight loss in the TGA curve confirms the encapsulation of FeNPs by the biomolecules of the extract which were dissociated by heat.