.The study appraised the sustainability of the improvement in post concession cargo throughput, revenue, ship traffic statistics and ship traffic volume performance of Onne seaport and developed benchmarks and planning models for sustenance of Onne seaport performance. Time series data of 10 years was gathered from the Nigerian ports authority on post concession cargo throughput, port revenue, ship traffic statistic and ship traffic volume performances of the seaport. The Arithmetic progression and series mathematical tool was used to analyze the data. It was found that; the post concession performance benchmark for each performance parameter of cargo throughput, port revenue, ship traffic volume and ship traffic statistics are C1 = 2,554,795 metric tons, R1 = 103.76MillionUSD, S1 =256,831,040 NRT, and V1 = 443 vessels respectively. The conditions for sustenance of the post concession cargo throughput, port revenue, ship traffic volume and ship traffic statistics performance of the port are: C1 + (n -1)d ≥2,554,795metric tons, R1 + (n -1)d ≥ 103.76MillionUSD, S1 + (n -1)d ≥ 256,831,040 Net Registered Tonnage and V1 + (n -1)d ≥ 443 respectively. Recommendations were proffered on the basis of the research findings.Keywords: sustainability-planning, bench-marking, post-concession, seaport, performance.