In this research study, a fully automated assembly sequence planner was developed, which automatically extracts geometrical information directly from STEP CAD files and then generates feasible assembly sequences with minimum assembly direction reorientations. The effectiveness of using the planner to reduce assembly time was also verified. The research study included three parts;In the first part of the research study, algorithms and software were developed for extracting geometrical information contained in STEP CAD files and for detecting potential collisions between parts during assembly along principal-axis assembly directions, based upon the extracted geometrical information. The developed software directly takes a STEP CAD file of a designed product assembly as input, and outputs six interference-free matrices representing collision information between parts in six principal assembly directions;In the second part of the research study, the algorithm developed in the first part was integrated into a genetic algorithm-based assembly sequence planner. The enhanced planner then was used to find assembly sequences with minimum reorientations automatically. The integrated assembly sequence planner directly takes a STEP CAD file of a designed product assembly as input, and outputs geometrically feasible assembly sequences requiring minimum reorientations;In the third part of the research study, a case study was conducted to verify the impact of assembly direction reorientations on assembly time, for both robot assembly and human operator assembly. Results of the case study show that, for both robot and human operator assembly processes, the number of reorientations in an assembly sequence has a significant impact on assembly time. The results support the primary research hypothesis that more assembly direction reorientations in a sequence require a longer assembly time. The case study helped verify and quantify the importance and effectiveness of using a fully automated assembly sequence planner to reduce the number of assembly direction reorientations in assembly sequence planning.