[Aim]  The external morphological characteristics of each Rhopalosiphum nymphaeae instar was documented to facilitate rapid instar identification for future studies. [Methods]  The morphological characteristics of R. nymphaeae were observed under the microscope, including measurements of body length, body width, and headshell width. The antennae, cornicle, cauda, hindfoot tibia, and wing base were measured after pressing. Differential and correlational analyses were carried out on the measurement data, followed by construction of box plots, to investigate the morphological variations across different developmental stages. [Results]  The antennae of 1st instar R. nymphaeae nymphs consisted of 4 or 5 segments. Similarly, 5 segments were observed in 2nd instar nymphs, while the antennae of 3rd instar nymphs had 5 or 6 segments. The antennae of 4th instar nymphs and older had 6 segments. No cauda or half-moon to triangle cauda were observed in 1st or 2nd instar nymphs, whereas half-moon to triangle cauda were observed in the 3rd and 4th instar nymphs. A cone-shaped cauda was observed in adult R. nymphaeae. In the winged morph of R. nymphaeae nymphs, differentiation of the wing base was observed to start at the 3rd instar. At this stage, the wing base extends from the first to the middle of the first ventral segment. By the 4th instar, the wing base extends further to the second to fourth ventral segment. Significant differences and correlations were observed between the morphological characteristics measured at different instars. However, the distribution of values for these characteristics tended to overlap between the different instar stages. The absence of an overlap may serve as a key indicator for age identification. For example, the wing base length of winged 3rd-4th instar nymph. A small amount of overlap can be used as an important indicator for instar identification, such as the body length of wingless 1st-4th instar nymphs. Partial overlapping values can serve as an auxiliary index for instar identification, such as the length of hindfoot tibia of wingless 3rd-4th instar nymphs. [Conclusion]  The number of antennae segments, cauda development, and the presence and length of the wing base can serve as key indices for determining the age of R. nymphaeae. The progressive increase in each index was correlated strongly with instar stage, providing a reliable reference index for determining R. nymphaeae age.