Peptidyl-prolyl isomerases (PPIases) accelerate proline peptide bond isomerization, affecting substrate protein function. In this study, through RNAi-based behavioral screening of PPIases in , we identified , termed peptidyl-prolyl isomerase-like 4 (dPPIL4), as crucial for circadian rhythm regulation. Knockdown of in clock cells lengthened the circadian rhythm period and decreased rhythmicity, accompanied by a significant reduction of core clock protein PERIOD (PER). knockdown downregulated transcription and reduced phosphorylation at Ser5 in the RNA polymerase II C-terminal domain, critical for transcription elongation. In addition, dPPIL4 stabilized Cullin1 of the Skp1-Cullin1-F-box protein complex, a key regulator of PER degradation. Our findings suggest that dPPIL4 supports high-amplitude PER oscillation by enhancing both synthesis and degradation processes in a timely manner. In conclusion, our study underscores the importance of high-amplitude PER oscillations in PER for robust circadian rhythms and highlights the critical role of dPPIL4 in this process.