Software-defined radio (SDR) is a kind of radio communication system which attempts to place much or most of the complex signal handling involved in receivers and transmitters into the digital style. As wireless technologies become ubiquitous, SDR are gaining popularity. In this work, we introduce the SDR platform USRP with emphasizes on hardware components, signal processing procedure and the supporting software. A radio spectrum monitoring system based on USRP and LabVIEW is designed and implemented, and web publishing of the real time spectrum is realized.