Aim: The aim of this in vitro study was to evaluate and compare the fit and fracture load of 3D printed and milled composite onlays. Materials and methods: An onlay preparation of a lower right molar was duplicated 28 times by means of 3D printing (Model 2.0, Nextdent, 3D systems, Soesterburg, the Netherlands). Fourteen onlays were milled (Brilliant Crios, Coltène, Whaledent AG, Switzerland) and 14 onlays were 3D printed (Nextdent C&B MHF, 3D systems, Soesterburg, the Netherlands). The marginal and internal fit were evaluated using the silicone-replica technique. For each impression, two sections were made in the silicone material. Each slice was photographed using a digital microscope and 13 measurements were performed on both slices using ImageJ. Next the onlays were cemented on their corresponding models (relyX Universal/SBU+, 3M ESPE, Seefeld, Germany) and tested in a universal testing machine (AGS-X, Shimadzu, Tokyo, Japan) to determine the fracture load. Results: The printed onlays showed a significant better marginal and internal fit (p<0.05). The marginal and internal fit for the 3D printed onlays was respectively 78µm (SD 23µm) and 222µm (SD 30µm) and for the milled onlays 100µm (SD 38µm) and 249µm (SD 55µm). In contrast, the milled onlays showed significant better results in terms of fracture load (p<0.05). Conclusion:Within the limits of this in vitro study, the printing process demonstrated a better fit, but with lower mechanical properties. Both the fit of the 3D printed and milled composite onlays were within the clinical acceptable limits.