Carrier mobility is an important factor in conjugated polymer opto-electronic devices. Its value is much smaller than that of semiconductor material, so time-of-flight (TOF) is often used to measure it. By setting up a temperature variable TOF detection system, the carrier mobility in polymer can be accurately measured. In principle and practice, some of the key factors affecting the accuracy of test are discussed, including the limit in the number of optical generated carrier, film thickness, pulse width, the external circuit response time, carrier relaxation period in a dielectric, as well as frequency of amplifier and detector. The accurate and reliable results can be achieved only when experimental parameters are selected appropriately.