The two leaders opened the pipeline's first stretch in Armenia in the town of Meghri, near the border with Iran.
Kocharian praised the "exemplary dyamism" in growing bilateral ties between Iran and Armenia:
"Just 15 years ago, in Meghri, there was nothing that could connect our countries," he said. "There were only the barbed-wire fences that symbolized the border of the Soviet Union. That's a fact. The bridge on which I greeted my Iranian counterpart today didn't exist. But just last year, 600,000 tons of goods were transported over that bridge alone."
Armenia seeks closer ties with Iran because of an economic blockade imposed by neighboring Azerbaijan and Turkey over the disputed Nagorno-Karabakh enclave.
The pipeline will also reduce Armenia's reliance on increasingly expensive Russian gas.
Once completed, the pipeline is expected to pump about 2.5 billion cubic meters of gas a year to Armenia.
An agreement to build the pipeline was signed in 1992, but construction began only in 2004.
(with material from IRNA, AP, AFP)