This study aimed at the following: (a) To evaluate the effectiveness of direct pulp capping (DPC) in children's primary molars using a combination of three antibiotics (3Mix-MP) as direct pulp capping materials compared with Calcium Hydroxide and (b) to assess the impact of pulp exposure diameter, its location and restoration extent on the success rate of the technique. This was a randomized double-blind, parallel, two-arm controlled trial. Forty-four primary molars with deep active caries, whose removal resulted in pulp exposures, were treated with direct pulp capping and were allocated randomly into two groups: Group A, DPC with 3Mix-MP; and group B, DPC with Calcium Hydroxide (CH). All teeth were restored with composite resin restorations. The exposures were classified according to their diameter, location and number of the filling surfaces. Clinical follow-ups were performed at intervals of 3months for 1year, and radiographical follow-ups were done after 6-12months. Data were assessed using Chi-squared test (χ2) at a 0.05 level of significance and binary logistic regression was performed to determine the effect of the characteristics of the carious lesion on success rates. The success rate of the technique was 54.5% in group A and 77.3% in group B, but the difference was not statistically significant. The pulp exposure diameter, its location and the number of the filling surfaces did not affect the success rate significantly. (a) Direct pulp capping in primary teeth achieved comparable success rates with no significant difference between 3Mix-MP and Calcium hydroxide. (b) The exposure location, its diameter and the extent of the restoration had no significant impact on the success rates of direct pulp capping in primary teeth. ISRCTN, ISRCTN13525994. Registered 31 October 2018 - Retrospectively registered, ( http:// www.isrctn.com/ISRCTN13525994 ).