Objective To study the effect of different optical impression methods in Cerec 3D/Inlab MC XL system on marginal and internal fit of all-ceramic crowns. Methods A right mandibular first molar in the standard model was used to prepare full crown and replicated into thirty-two plaster casts. Sixteen of them were selected randomly for bonding crown and the others were used for taking optical impression, in half of which the direct optical impression taking method were used and the others were used for the indirect method, and then eight Cerec Blocs all-ceramic crowns were manufactured respectively. The fit of all-ceramic crowns were evaluated by modified United States Public Health Service（USPHS） criteria and scanning electron microscope（SEM） imaging, and the data were statistically analyzed with SAS 9.1 software. Results The clinically acceptable rate for all marginal measurement sites was 87.5% according to USPHS criteria. There was no statistically significant difference in marginal fit between direct and indirect method group（P>0.05）. With SEM imaging, all marginal measurement sites were less than 120 μm and no statistically significant difference was found between direct and indirect method group in terms of marginal or internal fit（P> 0.05）. But the direct method group showed better fit than indirect method group in terms of mesial surface, lingual surface, buccal surface and occlusal surface（P
