In this paper, we consider a spectrum sharing cognitive radio(CR) network where a pair of twoway secondary users(SUs) is allowed to coexist with a primary user(PU) as long as the interference power received at the PU is below an acceptable limit. Under the interference power constraint, we investigate the effective capacity region for two-way SUs. In particular, we derive a closed-form solution for the power allocation problem to achieve the boundary points of an inner bound on the effective capacity region. We also propose a key parameter referred to as weighted-transmission-interference-ratio(WTIR), which plays a significant role in the power allocation policy. In the proposed power allocation policy, the exclusive transmission of SUs shall be performed if one SU’s WTIR is much larger than the others. That is, the one with much larger WTIR shall be allocated non-zero power and the other one shall be allocated zero power. Otherwise, if the difference of their WTIRs is not significant, the concurrent transmission shall be performed. That is, both SUs shall be allocated non-zero power. It is shown that a loose interference power constraint leads to the concurrent transmission in most situations, while a stringent interference power constraint leads to the exclusive transmission in most situations. It is also shown that for the stringent interference power constraint and loose delay quality-of-service(Qo S) constraint, the proposed power allocation policy is almost optimal and thus the obtained inner bound on the effective capacity region is tight. In this paper, we consider a spectrum sharing cognitive radio (CR) network where a pair of twoway secondary users (SUs) is allowed to coexist with a primary user (PU) as long as the interference power received at the PU is below an acceptable limit. Under the interference power constraint, we investigate the effective capacity region for two-way SUs. In particular, we derive a closed-form solution for the power allocation problem to achieve the boundary points of an inner bound on the effective capacity region. We also propose a key parameter referred to as weighted-transmission-interference-ratio (WTIR), which plays a significant role in the power allocation policy. In the proposed power allocation policy, the exclusive transmission of SUs shall be performed if one SU’s WTIR is much larger than the others. That is, the one with much larger WTIR shall be allocated non-zero power and the other one shall be allocated zero power. Otherwise, if the difference of their WTIRs is not significant That is, both SUs shall be allocated non-zero power. It is shown that a loose interference power constraint leads to the concurrent transmission in most situations, while a stringent interference power constraint leads to the exclusive transmission in The most situations. It is also shown that for the stringent interference power constraint and loose delay quality-of-service (Qo S) constraint, the proposed power allocation policy is almost optimal and thus the resulting inner bound on the effective capacity region is tight.