This article begins to introduce the relationship between latentdefect and ordinary wear and tear, then makes clear the source oflatent defect insured against and its essence. In order to understandthis concept of latent defect, the article lists out its difference in hullinsurance and marine transport. The article advocates the four con-ditions to form the damages arising from latent defect, and consid-ers if the insured fails to prove the four conditions, the underwriterwill undoubtedly refuse to indemnify and that undertaking his bur-den of proof is not required. The article concentrates on describinghow the insured and the underwriter to do when carrying their bur-den of proof, what onus of proof the parties to assume, and howthe nature of damages to be determined when the court settles thiskind of cases and so on.