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By Joseph Guyler Delva

PORT-AU-PRINCE, July 31 (Reuters) - Haiti's Senate on Thursday ratified economist Michele Pierre-Louis, President Rene Preval's third choice for prime minister, ending a three-month impasse after the last government was dismissed over food price riots.

Twelve senators voted in favor of the nomination of Pierre-Louis, a director of a foundation that provides libraries, youth education programs and women's networks in the impoverished Caribbean country. Five senators abstained and none voted against the nomination.

Pierre-Louis' nomination had already passed the lower house of parliament.

"The ratification of Ms. Pierre-Louis today takes us a step closer to the normalization of the political situation," said Sen. Eddy Bastien of the Democratic Alliance.

Two previous nominations for prime minister were rejected by lawmakers, hampering Preval's efforts to establish a stable democracy in a country that has known little but political upheaval and dictatorship since a slave revolt ended French rule more than 200 years ago.

The lack of a legitimate government also frustrated efforts by aid agencies and the United Nations to address overwhelming poverty that helped fuel violent riots over living costs in April in which at least seven people were killed.

Preval's last prime minister, Jacques Edouard Alexis, was fired by the Senate as a result of the violence, triggered by soaring food prices.

A plan to subsidize rice prices has helped to calm the frustrations but experts say Haiti's current stability and peace is fragile and could explode again into violence.

Preval, who took office in 2006 for his second term as president, previously nominated Ericq Pierre, an Inter-American Development Bank official, and Robert Manuel, a longtime friend and adviser, for prime minister. Parliament rejected both.

Pierre-Louis has been the director of FOKAL, a foundation supported by billionaire investor George Soros' Open Society Institute.

"She is a great woman with a lot of administrative skills and enjoying a lot of respect for her honesty and her social work," said Sen. Joseph Lambert of Preval's political movement Lespwa.

Pierre-Louis will have to appear before both legislative chambers in separate sessions to present a formal declaration of her policies before taking office.

She would become Haiti's second female prime minister after Claudette Werleigh, who was appointed by then-president Jean-Bertrand Aristide in 1995. Haiti had one female president, Ertha Pascal-Trouillot, who served in 1990. (Editing by Jim Loney)

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