The formal monarchy lasted from 1866 to 1947 when King Mihai (Michael) was forced to abdicate by the Communist government. He was allowed to return as a private citizen during the 1990s and certain privileges were restored including use of a Palace in Bucharest and recovery of family property.
King Carol I (1871-1916)
King Ferdinand (1916-1927) King Carol II (1930-1940) King Mihai (Michael)(1927-1930, 1940-1947)
There are many reference sites but a starting place is one that focuses on Queen Elisabeta, Queen Marie and Princess Ileana.
Another is the Maryhill Museum (Washington State) that maintains a museum wing devoted to Queen Marie. During the 1920s she visited the United States and was treated much as Lady Diana of England was in the 1990s.
Queen Marie 1875-1938, grand-daughter of Queen Victoria of England married to King Ferdinand; credited with influencing her husband, a Hohenzollern, to side with the Allies in WW I, a decision that led to the reunification of Transylvania into Romania in 1920
Prince Radu, husband of Princess Margarita, was designated a special representative of the Romanian Government. The members of the Royal family in Romania currently reside at the Elisabeta Palace (Bucharest).
Lft to rt: King Carol I, King Ferdinand, King Carol II