The speculative circuit is one kind of special parallelizing circuits; it may execute in parallel some of its functions out of their serial order and allows that although there are several data dependencies and control dependencies existing on those functions, some of whose external inputs (i.e. external dependencies) which are produced by other functions in a serial order may be produced by a predictive mechanism (or circuit). The predictive circuit cannot ensure that those speculative values are surely correct, and the results of speculatively executing functional units will be recovered when the speculative values are false; therefore, the suitable conflict management mechanism and the suitable data synchronization mechanism is needed when design a speculative circuit. In this paper, we present a new design model called 'Transactional Speculative Circuit Model' (TSCM), which is based on the concept of transactional memory (TM), to design the speculative circuits and the circuits with variable executing latencies and executing in parallel. Moreover, this model also can be used to design non-speculative parallelizing circuits. By using TSCM we had designed a new speculative MPEG-2 encoder with a good performance improvement.