The court ruled the proposed law was unconstitutional.
"The bill allows the distribution of seats in parliament to candidates nominated by parties on additional lists which have not been put to a vote," the court said in a statement.
It said that parliament would have to amend the law to bring it in line with the constitution.
The law was intended to make politicians more accountable to voters, by proposing a new mixed system, allowing voters to cast their ballots for individual candidates for half of parliament's seats, while party lists -- which are not put to the vote – would determine the rest.
The current system does not provide for the electorate to vote for individual candidates, and critics have said it has distanced politicians from the public.
The bill was also challenged by President Traian Basescu, who said it did not go far enough to clean up politics.
But Basescu’s own preferred option on electoral reform failed last month to be endorsed in a referendum because the turn-out did not reach the required level.
To keep its reform policy credible for investors, the government must find common ground with the IMF and look for a new arrangement, experts say.