In inline three cylinder engine, cylinder block is an integrated structure comprising the cylinders, coolant passages, intake and exhaust passages and ports, and crank case. The cylinder block is mostly fabricated by casting process. The surface irregularities in the top face of the casted cylinder block causes improper seating of cylinder head and surface finishing becomes essential. Hence the casted cylinder block was machined with series of operations such as rough milling, face milling, finish milling, honing, line boring, drilling, tapping and reaming operations. In face milling operation, milling machine requires special cutting tools to remove the surface irregularities of the cylinder block. In the vertical milling machine, 12.6 kg cutting tool is fitted and removed by manually and it leads safety problems to the workers. The manual tool changing method increases the idle time of the machine and hence a tool changing mechanism is required to overcome the above said problems. In this present work, tool changing mechanism is designed and fabricated for a vertical milling machine to perform top face milling operation. Hence a semi-automatic mechanism is suggested to minimize the time of tool replacement. The solid model of the semi-automatic mechanism was modelled using SOLIDWORKS and the stress analysis of the components of the tool changing mechanism was performed using ANSYS Workbench. The manual tool changing method increases the idle time of the machine and hence a tool changing mechanism is required to overcome the above said problems. The cutting tool replacement time is reduced as 4.10 minutes from 11.05 minutes.