Abstract: Due to uncertainties in borehole trajectory, it is very difficult to predict relatively positions of communicated wells accurately. To study errors in trajectories of two communicated wells, theories related to errors of conventional inclinometers, together with analysis over error data by software and performance parameters of communication tools were used in combination to highlight impact of borehole uncertainty on communicated well quantitatively. Relevant research conclusions can be used in trajectory design of communicated wells. In theory, these researches may enhance success rate of communication. Research results show that the extent of uncertainties in borehole trajectory may increase with well depth and inclination. Such impact shall be considered during design of distance-measuring points. By using the selection chart for ranging points generated through model construction, extent for potential ranging points can be determined in advance to simplify design procedures. Relevant research results may provide technical supports for trajectory design of communicated wells.