Spectrum sensing over imperfect channel condition can cause errors in primary signal detection. False alarm detects idle states as a busy channel and makes secondary user (SU) refrain from data transmission, while miss-identification detects busy states as an idle channel and causes collision to primary user (PU). This paper study the performance of opportunistic spectrum access (OSA) system over imperfect channel sensing with the restriction of maximum collision probability allowed (collision threshold) by PU. The successful transmitted bits over T slot is used to evaluate OSA performance and make comparisons to those ones without sensing error as function of number of slot. The throughput performance as a function of signal to noise ratio (SNR) is presented as well. According to the obtained results show that SU can improve the throughput performance by gaining the information from the previous slot over perfect and imperfect channel sensing. SU can maximize the number of bit transmitted within T slot transmission. Subsequently, imperfect channel condition can degrade the throughput achievement since it causes error in primary signal detection. © 2011 IEEE.