The purpose of this study is to analyze the effect of an access cavity design on stress distribution in an endodontically treated maxillary molar using a finite-element method. Two endodontic access cavities (EACs) were created: an ultraconservative 'ninja' EAC (NEAC) and a traditional EAC (TEAC). Four experimental groups were created for each type of cavity (group I: loads of 480 N and angle of 90 degrees; group II: loads of 480 N and angle of 45 degrees; group III: loads of 240 N and angle of 90 degrees; and group IV: loads of 240 N and angle of 45 degrees). The maximum von Mises stress values were calculated for the two types of cavities. It was found that the stress values for a TEAC were smaller than those of a tooth with an NEAC for all types of loading groups. The highest von Mises stress value was recorded as 702 MPa (group I) and the lowest von Mises stress value was estimated as 338 MPa (group IV) for a tooth with an NEAC. For a tooth with a TEAC, the highest and lowest von Mises stress values were recorded as 288 MPa (group II) and 142 MPa (group III), respectively.