Interface compliance verification plays a very important role in modern SoC designs. In order to perform a quantitative analysis of simulation completeness, adequate coverage metrics are mandatory. In this paper, we propose a finite state machine (FSM) based transaction-level functional coverage methodology for interface compliance verification. A language, State-Oriented Language (SOL), is developed to specify functional transactions mainly at the higher FSM level instead of lower logic or signal level. By utilizing SOL, it is simple and rigorous to specify interesting transactions from the specification FSM of the target interface protocol. Experimental results show that the proposed methodology can effectively improve the verification quality as well as increase the efficiency of regression verification.