Many studies have been conducted on climate change vulnerability assessments to develop adaptive strategies for climate change on a national or global scale. The development of an assessment tool for climate change on a process-level is necessary for evaluating vulnerability and to suggest an effective adaptive strategy in wastewater treatment plants (WWTP). Therefore, we proposed a vulnerability assessment index at the process-level in a WWTP to evaluate adaptive strategies for climate change in this study. The suggested process-level vulnerability assessment index is based on three performance WWTP indices: the effluent quality index (EQI), global warming potential (GWP), and operational cost index (OCI). Four different advanced WWTP processes were evaluated using the suggested vulnerability assessment index based on the A2 scenario, which is one of the carbon emission scenarios making predictions out to 2100 developed by the intergovernmental panel on climate change (IPCC). The adaptive strategies were evaluated at four conventional treatment processes to see the improvement of vulnerability of their processes, where the changes of their vulnerabilities are compared together. Suggested adaptive strategies in case studies showed that the process-level vulnerabilities were significantly decreased in the anaerobic/anoxic/aerobic (A2O) and Virginia initiative project (VIP) processes, especially during the flood and winter seasons. Therefore, it is expected that the proposed vulnerability assessment index can be useful as a decision-supporting tool for selecting the appropriate adaptive strategy for each process.