Sacral nerve stimulation (SNS) is a therapeutic medical procedure that stimulates the sacral nerve using an electrode inserted near the sacral nerve to treat the conditions of overactive bladder and fecal incontinence. The primary challenge in the SNS procedure is that the physician inserts a needle to place an electrode through the narrow structure of the sacrum while relying on fluoroscopic images. We developed a navigation system which relies on an off-the-shelf electromagnetic tracking device, preoperative CT images, and a graphical user interface for needle insertion. Subjects inserted the needle into targets in a free metallic environment and in a surgical environment with a sacral phantom model. The mean registration error in four trials was $0.90{pm}0.081mm$ in the free metallic environment and $1.16{pm}0.49mm$ in the surgical environment. The mean user error (UE) between the needle tip position and a target was $5.08{pm}1.27mm $in the free metallic environment and $4.26{pm}2.87mm$ in the surgical environment. We believe that our navigation system has potential to improve clinical sacral nerve stimulation treatment considerably. To support clinical application of this method, several remaining issues pertaining to the developed navigation system in an in vivo environment must be resolved.