This paper aims to improve the overall productivity of a windmill gear box manufacturing industry by effective implementation of lean principles. According to lean methodology, meeting the customer demand is done by effective utilization of various lean  tools like line balancing, kaizen implementations  and 5S principles, which makes the assembly process suitable for further continuous improvements.  Current State Value Stream Mapping (CSVSM) is plotted to understand the ongoing process in the assembly section of the gearbox manufacturing industry. After that the layout optimization of the assembly process is done to enhance the productivity by optimizing every section of assembly process to work under the takt time obtained from the customer demand. After the layout optimization, Future State VSM (FSVSM) is again plotted to study the improved process in the assembly line.