It is not merely an aggregation of static entities that a video clip carries, but alsoa variety of interactions and relations among these entities. Challenges still remain for a video captioning system to generate natural language descriptions focusing on the prominent interest and aligning with the latent aspects beyond observations. This work presents a Commonsense knowledge Anchored Video cAptioNing (dubbed as CAVAN) approach. CAVAN exploits inferential commonsense knowledge to assist the training of video captioning model with a novel paradigm for sentence-level semantic alignment. Specifically, commonsense knowledge is queried to complement per training caption by querying a generic knowledge atlas ATOMIC, and form the commonsense- caption entailment corpus. A BERT based language entailment model trained from this corpus then serves as a commonsense discriminator for the training of video captioning model, and penalizes the model from generating semantically misaligned captions. With extensive empirical evaluations on MSR-VTT, V2C and VATEX datasets, CAVAN consistently improves the quality of generations and shows higher keyword hit rate. Experimental results with ablations validate the effectiveness of CAVAN and reveals that the use of commonsense knowledge contributes to the video caption generation.