The chemoreception of butterflies has been comprehensively investigated in the last decades by electrophysiological methods. However, the molecular basis of olfaction in butterfly species has been poorly revealed. In order to clarify the molecular basis of chemoreception of Pieris brassicae and the role of olfaction in natural enemy avoidance of larvae, we identified the chemoreceptor genes in Pieris brassicae genome and antennal transcriptome and, quantified expression levels of odorant receptor genes in adult antennae, scanned the surface of sensilla, the chemosensory units and staining the neurons in antennae. We successfully established an overview of the larval chemosensory system at molecular, neuronal and morphological levels. We then selected a few odorant receptors that highly expressed in caterpillars and knocked out them by CRISPR/Cas9 system. Physiological, electrophysiological and behavioral changes were compared between wildtype and knockout insects to reveal the roles of the odorant receptors.