PURPOSE. To compare marginal and internal gaps of zirconia substructure of single crowns with those of three-unit fixed dental prostheses. MATERIALS AND METHODS. Standardized Co-Cr alloy simulated second premolar and second molar abutments were fabricated and subsequently duplicated into type-III dental stone for working casts. After that, all zirconia substructures were made using $Lava^{TM}$ system. Marginal and internal gaps were measured in 2 planes (mesial-distal plane and buccal-palatal plane) at 5 locations: marginal opening (MO), chamfer area (CA), axial wall (AW), cusp tip (CT) and mid-occlusal (OA) using Replica technique. RESULTS. There were significant differences between gaps at all locations. The $mean{pm}SD$ of marginal gap in premolar was $43.6{pm}0.4{mu}m$ and $46.5{pm}0.5{mu}m$ for single crown and 3-unit bridge substructure respectively. For molar substructure the $mean{pm}SD$ of marginal gap was $48.5{pm}0.4{mu}m$ and $52.6{pm}0.4{mu}m$ for single crown and 3-unit bridge respectively. The largest gaps were found at the occlusal area, which was $150.5{pm}0.5{mu}m$ and $154.5{pm}0.4{mu}m$ for single and 3-unit bridge premolar substructures respectively and $146.5{pm}0.4{mu}m$ and $211.5{pm}0.4{mu}m$ for single and 3-unit bridge molar substructure respectively. CONCLUSION. Independent-samples t-test showed significant differences of gap in zirconia substructure between single crowns and three-unit bridge (P<.001). Therefore, the span length has the effect on the fit of zirconia substructure that is fabricated using CAD/CAM technique especially at the occlusal area.