Filling the substrate in the bag is the process in mushroom cultivation that requires the most labor. This need techniques and skills to be done properly. Consequently, the machine becomes necessary. The main objective of this study was to develop operational procedure of the existing Bag filling machine for Oyster mushroom substrate in Pampanga State Agricultural University (PSAU) Mushroom Center and determine the capacity, efficiency and economic viability. The bag filling machine undergone preliminary testing to establish the unit's operational procedure. The machine can only be operated on sawdust substrate, according to observations made during the preliminary testing. Data on the machine's capacity, losses, and efficiency with varying number of operators were gathered during the final testing. The filling capacity of the machine was 175 bags/h for one operator while it took 207 bags/h for two operators. The filling losses of the machine was 2.69% for one operator and 2.57% for two operators. The filling efficiency of the machine was 46.47% for one operator while it was 48.79% for two operators. Machine efficiency is at 97.37%. According to the results, the machine was found more economically viable when it is operated by one person, which yield to net income of Php 264,146.4 and the payback period of 84 days compared to Php 201,532.8 payback period of 81 days for two operators.