An ICP-MS method using a micro-flow injection (μ-FI) sample introduction system was developed for measuring ²³⁷Np and ^239,240,241Pu in urine samples for sensitive and rapid population monitoring following a radiological or nuclear accident. Good selectivity from the chemical separation method allowed the determination of ²³⁷Np together with Pu isotopes using ²⁴²Pu as a tracer. Significant improvements in ICP-MS sensitivity and detection limit were achieved using the μ-FI sample introduction and the desolvation techniques. The method developed has been successfully applied to a set of human urine samples spiked with Pu isotopes and a set of rat urine samples with metabolized Pu isotopes from research experiments.