The purpose of this paper is to analyze the mechanism of the spalling, which is tensile cracking that leads to the removal of the coating under sliding contact between a spherical diamond pin and a flat coated disk. For this, the SEM-VTR-Tribosystem (pin-on-disk Tribosystem combined with a scanning electron microscope (SEM) and a video tape recorder (VTR)) was developed and introduced in the observation of the tensile cracks and the spalling caused by them. Based on the experimental observation and linear elastic fraction mechanics, a contact mechanic model was proposed to account the critical condition for the propagation of a pre-present flaw. The analysis by the experiments with the SEM-VTR-Tribosystem and the calculations by the contact mechanic model shows that a parameter sigma(xx,max) square-root C(pre)/K(IC,f) (sigma(xx,max) is the maximum tensile stress on coating surface, C(pre) is that length of a pre-present flaw in coating and K(IC,f) is the coating fracture toughness) can be used to predict approximately the onset of the spalling of the ceramic coating under sliding contact.