JERUSALEM (AP) _ President Chaim Herzog on Sunday reduced the prison sentence peace crusader Abie Nathan is serving for meeting with the PLO, clearing the way for his release by April 9.

Herzog cut one year from the 18-month sentence Nathan began last October for a 1990 meeting with PLO Chief Yasser Arafat in Tunis.

Nathan was twice convicted under a 1986 amendment to Israel's anti-terror law that prohibits meetings with members of terrorist groups. Israel considers the Palestine Liberation Organization to be a terrorist group. Conviction carries a maximum three-year prison term.

The president's action came after Nathan promised the president in writing that he would obey the laws of Israel if his sentence was reduced.

Giora Pordes, Herzog's spokesman, said that the president decided to release Nathan because he committed ''from here on only to act in accordance with the law.''

Justice Minister Dan Meridor also recommended that Nathan's sentence be reduced.

Nathan, 64, a former fighter pilot who now runs a pirate radio station, has led a one-man campaign for three decades to get talks going between Israel and the Arabs.

He is best known for making a dramatic trip in a private plane to Cairo in 1966 to try to convince Egypt to make peace with Israel.

Nathan argued that it was hypocrisy to keep him in prison while U.S.-backed Israel-Arab peace talks are under way.

Nathan's lawyer, Pinchas Marinsky, said Nathan turned to the president and agreed not to break the law again because ''Israel began direct negotiations with the Palestinians.''

Marinsky said he believed Nathan would continue to fight ''in every way possible'' to have the anti-terror amendment repealed.

Nathan has been in prison since last Oct. 10. He previously served four months in jail on a 1989 conviction for meeting Arafat and other PLO leaders.