Objective      To determine the effect of different luting spaces on marginal and internal fit of Cerec Blocs all-ceramic crowns in InLab MC XL system.       Methods      A right maxillary first molar in the standard model was prepared and subsequently replicated into silicone rubber negative mold. Respectively

30 preparations of all-ceramic plaster and die stone were irrigated. Thirty all-ceramic plasters were used for optical impression

and then 30 three-dimensional digital images were obtained and randomly and equally divided into 6 groups

with their luting spaces were set to 0 (group A)

10 (group B)

20 (group C)

30 (group D)

40 (group E) and 50 μm (group F) respectively by computer software. In addition

30 die stones were prepared for bonding of the all-ceramic crowns. After being bonded

embedded and slitted

these specimens were scanned by stereo microscopy.       Results      The marginal fit was ranging from 30 to 120 μm and the internal one from 30 to 300 μm in groups C and D. There were statistically significant differences between groups C and D in the marginal fit（P0.05）.  The fit of occlusal surface was better than other parts of the internal fit in groups C and D.       Conclusion      All-ceramic crowns can obtain the marginal and internal fit which meets clinical requirements when luting spaces are 20 and 30 μm. The crowns with luting spaces of 30 μm is better than that with 20 μm in the marginal fit

but both are worse in the fit of occlusal surface than other parts of internal fit.