Electronic properties of singlecrystalline La3Ir3O11, which hosts strongly spin–orbit entangled 5d electrons on the geometrically frustrated KSbO3-type structure, have been investigated. The magnitude of spin–orbit coupling in the 5d band was estimated from the X-ray absorption spectrum. Electrical resistivity and Hall measurements reveal a crossover from dominant hole carriers at room temperature into dominant electron carriers at low temperature, indicating that La3Ir3O11 is a semimetallic state. A small anomaly appears in the specific heat and electrical resistivity at around 3.5 K, suggesting a possible magnetic transition. On applying high pressures up to 13 GPa, the state changes into an incoherent state characterized by a small temperature dependence, and the possible magnetic transition has been suppressed.