A remote online machine monitoring system is developed across a desktop PC and mobile devices to monitor and control machine’s working conditions. A server-client network structure is constructed for real time communication between server and client modules. Within the server module, data I/O (input/output) communication is utilised to acquire machine condition data and to pass control commands to the machine-controller interface; feature extraction and decision making are applied to detect machine’s operating status. The server and client modules allow mobile users to remotely access the server via PDA and Wireless Wi-Fi connectivity to regulate the machine conditions by tuning the controller parameters. Remote communication between the modules with different platforms/operating systems is implemented by combining the Web Service application program interface with MXML-based cross-platform programming standard. The system developed is validated by a case study of monitoring the working conditions of an air-conditioning system.