MicroRNA-mediated post-transcriptional regulations are increasingly recognized as important components of the circadian rhythm.Here we identify microRNA let-7, part of the Drosophila let-7-Complex, as a regulator of circadian rhythms mediated by a circadian regulatory cycle.Overexpression of let-7 in clock neurons lengthens circadian period and its deletion attenuates the morning activity peak as well as molecular oscillation.Let-7 regulates the circadian rhythm via repression of CLOCKWORK ORANGE (CWO).Conversely, up-regulated cwo in cwo-expressing cells can rescue the phenotype of let-7-Complex overexpression.Moreover, circadian prothoracicotropic hormone (PTTH) and CLOCK regulated 20-OH ecdysteroid signaling contribute to the circadian expression of let-7 through the 20-OH ecdysteroid receptor.Thus, we find a regulatory cycle involving PTTH, a direct target of CLOCK, and PTTH-driven miRNA let-7.