GaN based light-emitting diodes (LEDs) have been attracting a great deal of interest due to their capability in emitting a spectrum from ultraviolet to green and their applications in traffic signals, displays and solid-state lighting. However, the high efficiency of LED is still obstructed by light-extraction efficiency. In this work, we propose that light-extraction efficiency of GaN-based blue LED should be improved by a self-assembled monolayer of polystyrene spheres. The GaN-based LED grown on sapphire substrate emits the light mainly from the indium tin oxide (ITO) transparent electrode. And the hexagonal closely-packed polystyrene sphere monolayer is formed onto the ITO layer. In order to study the light-extraction efficiency affected by the size of nanosphere, nanosphere monolayers of different sizes are prepared onto the ITO layer, and the diameters of the polystyrene spheres are 250, 300, 450, 600 and 950 nm, respectively. The electroluminescence results show that using polystyrene sphere monolayer can improve the light-extraction efficiency compared with using the conventional LEDs, and the light-extraction efficiency reaches a maximum when the average size of spheres (450 nm) approximates to the wavelength (465 nm) of that light. The light output power of the LED with polystyrene sphere of the optimum size is experimentally enhanced by 1.34 and 1.25 times under the injection currents of 20 and 150 mA, respectively. In order to explain the physical mechanism of the light-extraction enhancement, we carried out the three-dimensional finite difference time-domain simulation thereby calculate the transmission spectrum of the structure. The results of simulation show that the incident light beyond the critical angle can be partly extracted when the surface of LED has a polystyrene sphere monolayer, leading to an enhanced light-extraction efficiency. So the nanosphere monolayer acts as a two-dimensional diffraction lattice which behaves as a light scattering medium for the light propagating in a waveguiding mode within the LED. Furthermore, the polystyrene nanosphere has the advantages of low-cost and high-precision, and is very suitable for large area preparation on LEDs. So this method is a simple and cost-effective method to improve the light-extraction efficiency from LED.