This paper presents an ultra-wideband spectrum monitoring system using a software defined radio platform. The system has been developed by using an Ettus X310 universal software radio peripheral (USRP) for data acquisition and a laptop for displaying the graphical user interface. The system can operate in two modes with two different daughter boards installed. One of the operation modes allows simultaneously monitoring up to two different channels, each with a maximum sampling bandwidth of 200 MHz, or simultaneously monitoring up to four different channels, each with a maximum sampling bandwidth of 100 MHz. The other operation mode allows monitoring a single channel with a maximum sampling bandwidth exceeding 300 MHz. Furthermore, unlike the widely adopted GNU Radio software, the proposed system software is friendly to the Microsoft Windows operating system.