<p>This study aimed to compare the shaping ability of ProTaper Ultimate, ProTaper Gold, and Reciproc Blue in the mesiobuccal canals of maxillary first molars using micro-computed tomography. Thirty extracted maxillary first molars with Vertucci Type IV configuration and curvatures between 10° and 20° were allocated into three groups (<i>n</i> = 10): ProTaper Ultimate, ProTaper Gold, and Reciproc Blue. All specimens underwent pre- and post-instrumentation micro-computed tomography scanning. Canal transportation, centering ratio, remaining dentin thickness, canal volume, and surface area were evaluated at predefined canal levels. Data were analyzed using one-way analysis of variance (<i>α</i> = 0.05). A significant difference was observed only at the 5&#xa0;mm level of the MB1 canal, where Reciproc Blue showed a higher centering ratio than ProTaper Gold (<i>p</i> &lt; .05). ProTaper Ultimate resulted in thinner remaining dentin in the furcation region compared with ProTaper Gold (<i>p</i> &lt; .05), although all systems preserved more than 0.3&#xa0;mm of dentin thickness. MB2 negotiation failures and instrument separation were most frequently observed in the ProTaper Gold group. Although localized differences were observed, all systems preserved canal anatomy with minimal transportation. Reciproc Blue demonstrated superior apical centering, whereas ProTaper Ultimate resulted in slightly thinner dentin in the mesial furcation area. This micro-computed tomography study provides comparative evidence on the shaping behavior of three heat-treated nickel–titanium systems in moderately curved mesiobuccal canals, and the observed differences in centering ability and dentin preservation may assist clinicians in selecting safer instrumentation strategies for complex molar anatomies.</p>