Circadian clocks are comprised of self-sustained transcriptional/translational feedback loops, which regulate the rhythms of physiology and behavior in mammals. CLOCK-interacting protein, Circadian (CIPC), has been indicated as an additional negative-feedback regulator of the circadian clock in vitro, although its physiological roles in circadian clock are unknown. Here, we generated Cipc homozygous knockout (Cipc (-/-)) mice and assessed the resultant circadian phenotypes. Surprisingly, the mRNA expression profiles of core clock genes in the liver of Cipc (-/-) mice showed no significant differences from that in wild-type mice except for Per1. Cipc (-/-) mice displayed normal locomotor rhythm and entrained activity pattern in both 12:12 light-dark cycle and constant dark cycle. Furthermore, deletion of Cipc in lungs and adipose tissues did not influence their peripheral clock. The results from this work provided more conclusive data suggesting that CIPC is not critically required for basic clock function.