OBJECTIVES: To evaluate the accuracy of direct restorations using transparent vinyl polysiloxane (VPS) indices with different thicknesses for composite resin injection technique. METHODS: Thirty-six standard resin teeth of tooth #11 were divided into 3 groups based on the index thickness including 2, 4, and 6 mm (n = 12). VPS indices were fabricated using customized trays corresponding to each thickness. Tooth defect was created by sectioning 4 mm of the crown, which was then restored using the composite resin injection technique under the VPS index. Pre- and post-treatment scans were fitted, generating a deviation distribution map of tooth #11. Overall root mean square (RMS) and distance deviations along the central longitudinal section were recorded for accuracy evaluation. One-way ANOVA with Tukey post hoc comparisons and the Levene test were used for statistical analysis (α = 0.05). RESULTS: Both mean and standard deviation values of RMS for injected restorations decreased as index thickness increased, with no significant differences observe between 4-mm and 6-mm index groups. Distance deviations of the injected restorations were generally less than 0 in all groups, and deviations of the middle third of the lingual surface significantly exceeded 50 μm in 2-mm index group. CONCLUSIONS: The accuracy including trueness and precision of direct restorations using the composite resin injection technique improved with increasing index thickness, but no significant differences were found between the 4-mm and 6-mm indices. Besides, all the results were within the clinically acceptable error values for the 4-mm and 6-mm indices. CLINICAL SIGNIFICANCE: For achieving clinically acceptable accuracy in restorations using the composite resin injection technique, a VPS index thickness of 4 mm is both the minimum requirement and sufficient, ensuring satisfactory esthetic and functional outcomes.