This research work aimed to provide an insight into the performance of colonoscopes with different diameters influenced by patients' gender through numerical simulation. Firstly, several colon simulation models considering the structural differences between female and male large intestines were established. The simulations of the insertion of the colonoscopes with different diameters into the colon models were subsequently conducted. The corresponding colon deformations were investigated. The deformations were used to predict patients' pain. It was observed from the simulation results that small-caliber (SC) colonoscopes could be more helpful in reducing patient pain for female patients when compared with standard colonoscopes (SDC). To further explore the possible reasons, motivated by the interaction between drill string and well in petroleum industry, several insertion simulations within fixed boundary of colons were performed. Two quantities, including the buckled length of a colonoscope and the contact force between the buckled colonoscope and the colonic wall, which were positively correlated with the deformation of colon, were investigated. It was found that, compared to the SDC, the SC colonoscopes tended to cause much lower contact force but easily approach buckled length for female colons. Thus we concluded that the SC colonoscopes were efficient in reducing female patients' discomfort during colonoscopies.