Planning and simulation in assembly processes and whole production systems by computer aided systems is currently an advantage of big major companies. However, computer aided technologies in field of planning and simulation of production systems and assembly systems are future of every one company if they want to remain competitive. Computer aided systems in assembly and simulation of production processes help to save time thus financial resources needed for planning and optimization of every production systems and sub-systems. This kind of computer aided systems are helping to eliminate errors in production processes and assembly systems before they can happen in reality. This paper deals with possibilities of improvement in production process planning by use of appropriate software. In this case we will use Tecnomatix Plant Simulation by SIEMENS company which. Use of this software is aimed for planning and optimizing of production systems with high level of complexity.