This paper investigates the consensus problem of single integrator multi-agent systems with unbounded time-varying transmission delays and fixed directed topology. A distributed controller is proposed taking into consideration the unbounded transmission delays. It is shown that the consensus problem can be solved via the proposed controller under the spanning tree assumption and the nocycle assumption. The authors characterize the no-cycle assumption and prove the consensus of the concerned multi-agent system with an induction procedure. Two simulation examples are provided, one to illustrate the effectiveness of our result and the other to show the necessity of the no-cycle assumption.