The German government is among the treaty's strongest backers
German ministers have told judges in the country's constitutional court that the Lisbon Treaty will improve democracy and EU decision-making.
They were speaking on the first day of a hearing at the constitutional court to decide whether the treaty violates the German constitution.
The controversial treaty - not yet ratified by all 27 member states - is aimed at strengthening EU institutions.
The German court ruling is expected to come in a few months' time.
The treaty was rejected by Irish voters in a referendum last June. A new referendum is planned for this autumn, with the Yes camp hoping that new EU "guarantees" on Irish sovereignty will be enough to persuade voters to back it.
The Czech Republic and Poland have not yet ratified the treaty either. All member states have to ratify it for it to take effect.
Addressing the court in Karlsruhe, the German Foreign Minister, Frank-Walter Steinmeier, said the treaty would boost democracy in Europe and give national parliaments a greater role in the EU legislative process.
"The Treaty of Lisbon expressly strengthens the democratic fundamentals of the European Union," he said.
Interior Minister Wolfgang Schaeuble said the treaty did not compromise German sovereignty.
Germany's parliament has already approved the treaty, but President Horst Koehler has delayed signing it until the court gives its ruling.
Opponents say the treaty is part of a federalist EU agenda that threatens national sovereignty. The also say it differs little from the ill-fated EU constitution, rejected by French and Dutch voters in 2005.
The case against the treaty was brought by Peter Gauweiler, a conservative Christian Social Union (CSU) member of the Bundestag (lower house), and backed by some Left Party MPs.
A court ruling could be further delayed by a separate anti-Lisbon complaint brought by Franz Ludwig Graf von Stauffenberg, a lawyer and former CSU deputy.