Today, the competitive world requires modern assembly industries to restructure their processes in order to maximize efficiencies. With evidence gathered in two assembly lines, one manual and the other semi-automated, this work proposes process mapping as a starting point for the application of a costing system based on activities. For this purpose, data are collected through empirical observation techniques and individual interviews. Afterwards, this information is transferred to process sheets under ISO 9001 standard and BPMN notation (Business Process Model and Notation) and analyzed by means of statistical techniques. The work concludes that processes mapping facilitates the application of activity-based costing systems, and processes optimization for a better performance of the organization.