Femtocell networks have been considered to be a key technology to improve the capacity of cellular systems. However, femtocell networks are usually deployed in indoor environment. Therefore, femtocell placement problems are the most important issue to solve in planning a wireless network, since the conventional manual planning processes is highly inefficient. This paper presents an approach for solving femtocell placement problems in an indoor environment using particle swarm optimization (PSO). The main objective is to select the best position of the femtocell so as to maximize indoor downlink capacity. In a 100 trial study, optimization improved the average SNR up to 3.5 dB. It is shown that the approach is efficient and effective. In conclusion, the approach algorithm is able to improve the throughput of femtocell networks.