The Aim of the present study was to compare the effect of reciprocating and rotating enlarging instruments on the shaping geometry of distal canals of mandibular molars when used in a modified contracted access cavity (CECDW). Materials &Methods: Fifty-four mandibular molars with separate mesial and distal roots were accessed based on the CECDW design. The teeth were randomly divided into three groups (n=18) and were assigned for the preparation of distal canals using continuous rotation with OneShape or Reciprocation with Reciproc and WaveOne. Micro Computed Tomography (MCT) was utilized to compare shaping parameters: canals' volume, surface area, and stracture model index (SMI). The percentage change in volume, surface area, and canal transportation between the two groups was compared using an independent t test. The significance level was set at P ≤ 0.05. Results: in all tested instruments, canals' volume and surface areas showed significant increase as compared to the unprepared ones. The highest increase in both parameters was detected in WaveOne P=0.004 and P= 0.043 respectively, followed by Reciproc and OneShape in descending order. All three instruments resulted in a significantly rounder preparation compared to pre-instrumentation SMI <0.001. Conclusions: CECDW and experimented instruments did not show significant improvement in shaping geometry of distal canals in mandibular molars. WaveOne performed significantly better than Reciproc and OneShape in respect to increasing the canals' volume and surface area. All tested instruments working through CECDW resulted in shift of the SMI towards more round preparation geometry. However, OneShape showed the best maintenance of SMI.