Excluding, of course, Pope Benedict XVI, head of state of Vatican City?
Beatrix of the Netherlands is a widow (or did you mean never-married?).
Either/or.
Jarosław Kaczyński – prime minister of Poland (and the twin brother of the president of Poland, who is married).
This chick is married, but I hear you can get away with rubbing her neck and shoulders.
Sailboat
Dipendra, the murderous three-day King of Nepal, was unmarried… which is thought to have been a factor in his rampage: Dipendra of Nepal - Wikipedia
Historically, of course, you have Queen Elizabeth I of England, and President James Buchanan (never married) and Grover Cleveland (married in the White House).
Ariel Sharon was a widower, twice over - a pair of sisters, in fact. His first wife died in a traffic accident in the 60’s, he married her sister some time afterwards, and she died of cancer about ten tears ago.
Junichiro Koizumi, the former Japanese PM who just left office, was divorced and never remarried.
Edward Heath , UK Prime Minister during the 1970’s was unmarried
It sounds like you mean current heads of state, but of past U.S. Presidents, James Buchanan was unmarried, and I believe there were others who were unmarried during at least part of their term in office.
Current president of Finland (Tarja Halonen ) was not married when she was elected.
Michelle Bachelet of Chile is separated.
Joaquín Balaguer of the DR was a lifelong bachelor (no, really) who fathered a number of children out of wedlock.
Leonel Fernández, also of the DR, was between marriages during his first presidential term (1996-2000).
Pierre Elliott Trudeau, Prime Minister of Canada (1968-1979; 1980-1984) was unmarried when first elected to office. He married Margaret Sinclair in 1971, but the marriage didn’t last - they finally divorced in 1984, around the time he left office.
For a period of about 27 years, Canada had Prime Ministers who were not married.
William Lyon Mackenzie King, served three terms as Prime Minister of Canada (1921-1926; 1926-1930; 1935-1948). King was never married.
Nor was Richard Bedford Bennett, Prime Minister from 1930 to 1935, in between King’s second and third term.
Arthur Meighen served a short term as Prime Minister in 1926 for three months in between King’s first and second terms. Meighen was married.
So except for those three months in 1926, Canada’s Prime Ministers were unmarried from 1921 to 1948.