  We report studies on pressure dependence of superconducting transition
temperature (Tc) of LaFeAsO, LaFeAs(O0.5F0.5) and LaFeAs(O0.89F0.11) samples.
In-situ resistance measurements under high pressure showed that the Tc of these
three compounds increases with pressure initially, reaches a maximum value and
then decreases with further increasing pressure, although the Tc at ambient
pressure are different. The onset Tc of LaFeAsO is ~50 K at 1.5 GPa, which is
the highest record in La-based oxypnicited system. The significant change in Tc
induced by pressure is attributed to the orbital degeneracy and the electron
density of state at the Fermi level.
