A novel method of synthesizing aqueous suspensions of monodisperse thermo-responsive poly(N-isopropylacrylamide) nanogel particles of varying sizes in the deci-micron range viz., controlled drop addition method (CDAM) by modifying the one-pot method has been developed. Unlike the conventional one-pot method which lacks control on the growth of the nuclei, this method controls the nuclei growth, thereby controlling the size polydispersity. The nuclei growth can be controlled by varying the drop rate as well as the ratio of pre-gel solution. The study showed that an optimum drop rate is desired for achieving narrow size distribution. The synthesized suspensions have been characterized for their particle size and size polydispersity using dynamic light scattering technique. The resultant particles fall well in the deci-micron regime with narrow size distribution. A plausible mechanism backed by atomic force microscopy images, governing the growth and size polydispersity is discussed. Implication of these monodisperse PNIPAM nanogel particles as carriers in drug delivery and as photonic crystals is briefed.