[Objectives]  To determine the larval instars of Chironomus flaviplumus Tokunaga. [Methods]  Egg masses were artificially cultured in the lab and larvae sampled periodically for analysis. Eleven morphological metrics, including head capsule length, the length from the tip of the median teeth of the mentum to the leading coronoid, head capsule width, mentum width, the width of the median teeth of the mentum, antenna length, length of the basal antenna segment, the length of antenna segments 2-5, mandible length, and ventromental plate length and width, were measured. The optimal morphological metrics were chosen according to the Crosby growth rule, their frequency distribution and regression analysis. [Results]  The larval stage of C. flaviplumus is comprised of 4 instars and mentum width is the best morphological metric for identifying these. The average mentum width (μm) of instars 1^st-4^th were (30.9008±0.3253), (52.2114±0.7172), (98.6165±0.6146), and (171.2985± 0.5582), respectively. The linear regression equation of the relationship between mentum width and instar number is y=0.1971x³+11.365x²-14.163x+33.502，R²=0.9827. The length from the tip of the median teeth of the mentum to the leading coronoid, the width of the median teeth of the mentum, and the length of the mandible, can be used as auxiliary morphological metrics for identifying instars. Estimated average stadia of instars 1^st-4^th based on mentum width were 2-3, 2-4, 5-6 d and 7 d, respectively. The overall duration of the larval period was 16-20 d. [Conclusion]  The larval stage of C. flaviplumus is comprised of 4 instars and mentum width is the best morphological metric for identifying the larval instars of this species.