This paper attempts to discover the effect of emission trading and joint implementation policies on total CO2 abatement cost when they are applied simultaneously. Studies show that emission trading policy is the most cost-effective approach for reducing CO2 emissions. Joint implementation is thought to reduce abatement costs when the investing country's abatement cost is higher than that in the host country. This study concludes that joint implementation curtails the host country's domestic abatement opportunity. Therefore, when joint implementation is applied, benefits obtained by the investing country must be higher than the opportunity cost lost in the host country. Further, this study also concludes that the two policies applied simultaneously cannot effectively reduce CO2 cost in the monopoly market. Apart from this, total abatement costs for the two involved countries vary with different initial CO2 reduction commitments.