AbstractThis paper examines the splicing loss due to axis misalignment in the splicing of single‐mode optical fibers using direct core monitoring and discusses the methods for reducing such loss and the techniques for estimating it. Axis misalignment is strongly dependent on the offset between the monitoring location and the actual position due to the fiber lens effect and core eccentricity as well as on the precision of the core monitoring system. Taking into consideration the indeterminateness of the relative position of the fibers, we analyze this relationship by means of a statistical technique and, using a direct core monitoring system with transmitted light, verify the appropriateness of the theory by means of experimental studies. Further, it is shown that it is necessary to reduce the alignment error due to core eccentricity and that it is possible to align the cores to a precision of better than 0.5 μm by correcting the core center position from the observed eccentricity. In addition, by considering the surface tension effects during fusion we can explain the relationship to axis shift before and after splicing and, if the axis alignment precision is given prior to splicing, we can estimate the splicing loss distribution. We also show that it is theoretically possible to achieve an average splicing loss of less than 0.1 dB with single‐mode fibers using this direct core monitoring method when the axes are aligned to within 0.5 μm.