We investigate the collective and individual emissions from In 0.5Ga0.5As quantum dots (QDs) in single-defect photonic crystal nanocavities. The cavity mode collectively excited by the QD ensemble shows a pair of dipole-like modes with definite linear polarizations. Single exciton emission lines are resolved under low excitation power. By monitoring the power dependence of individual QD emissions, a nearly tenfold light enhancement for on-resonance QDs is observed. The polarization states of individual QDs are also investigated. It is found that either a pure dipole mode or a mixture of both dipole modes could be excited by an individual dot. These behaviors are attributed to the random distribution of QD location in the nanocavity.