The cold neutron research facility project at the Korean HANARO research reactor was launched in 2003. A state-of-the-art small-angle neutron scattering instrument was selected as a top-priority instrument. An analytical calculation was used to obtain the optimum instrument design and the appropriate range of scattering vectors of the instrument. Then, in order to optimize the instrument layout and to enhance the instrument's performance, a Monte Carlo simulation was performed. The Q range [where Q is the magnitude of the scattering vector given by (4/)sin(/2), is the scattering angle and is the wavelength] of the instrument will extend from 0.0008 to 1.0 Å^-1. The instrument layout was optimized based on simulation results. The maximum flux at the sample position can reach about 5.5 × 10 ⁷ n cm^-2 s^-1.