An analytical method was developed for simultaneous determination of ultratrace level plutonium (Pu) and neptunium (Np) using iron hydroxide coprecipitation in combination with automated sequential injection extraction chromatography separation and accelerator mass spectrometry (AMS) measurement. Several experimental parameters affecting the analytical performance were investigated and compared including sample preboiling operation, aging time, amount of coprecipitating reagent, reagent for pH adjustment, sedimentation time, and organic matter decomposition approach. The overall analytical results show that preboiling and aging are important for obtaining high chemical yields for both Pu and Np, which is possibly related to the aggregation and adsorption behavior of organic substances contained in urine. Although the optimal condition for Np and Pu simultaneous determination requires 5-day aging time, an immediate coprecipitation without preboiling and aging could also provide fairly satisfactory chemical yields for both Np and Pu (50-60%) with high sample throughput (4 h/sample). Within the developed method, Pu-242 was exploited as chemical yield tracer for both Pu and Np isotopes. Pu-242 was also used as a spike in the AMS measurement for quantification of (PU)-P-239 and Np-237 concentrations. The results show that, under the optimal experimental condition, the chemical yields of Np-237 and Pu-242 are nearly identical, indicating the high feasibility of Pu-242 as a nonisotopic tracer for Np-237 determination in real urine samples. The analytical method was validated by analysis of a number of urine samples spiked with different levels of Np-237 and (PU)-P-239. The measured values of Np-237 and (PU)-P-239 by AMS exhibit good agreement (R-2 >= 0.955) with the spiked ones confirming the reliability of the proposed method.