The recent advances in superconducting nanowire single-photon detector (SNSPD or SSPD) technology has enabled long distance quantum key distribution (QKD) over an optical fiber. We point out that the performance of SNSPDs play a crucial role in achieving a secure transmission distance of 100 km or longer. We analyze such an impact from a simplified model and use it to interpret results from our differential-phase-shift (DPS) QKD experiment. This allows us to discuss the optimization of the detection time window and the clock frequency given the detector characteristics such as dark count rate, detection efficiency, and timing jitter.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.