Software-Defined Radio (SDR) is a promising radio technology that implements radio communication functionalities in software instead of hardware. Advantages of a SDR system are reconfigurability and flexibility whereas disadvantages are low throughput and high latency. Our research focuses on analyzing and measuring latency of a SDR system based on GNU Radio with three versions of Universal Software Radio Peripherals (USRP) devices - USRP1, USRP2 and USRP E100. This research identifies the sources of the latency and quantifies them by using both analytical and experimental methods. For the analysis, we identify all types of buffers in the SDR platforms and estimate the time that these buffers contribute. The corresponding results are compared with experimental measurement, which estimates the round-trip time between two SDR systems. This can be done by using TUN/TAP components and GNU Radio tunnel program. Alternatively, we also propose a method called hwlatency to measure the latency in hardware side. We believe this study offers the better understanding of the latency in SDR platforms and will lead to correct implementations of high-level network protocols for SDR systems.