Abstract: Pentatricopeptide repeat (PPR) proteins, composing one of the largest protein families in plants, are involved in RNA binding and regulation of organelle RNA metabolism at the post‐transcriptional level. Although several PPR proteins have been implicated in endosperm development in rice (Oryza sativa), the molecular functions of many PPRs remain obscure. Here, we identified a rice endosperm mutant named floury endosperm 18 (flo18) with pleiotropic defects in both reproductive and vegetative development. Map‐based cloning and complementation tests showed that FLO18 encodes a mitochondrion‐targeted P‐type PPR protein with 15 PPR motifs. Mitochondrial function was disrupted in the flo18 mutant, as evidenced by decreased assembly of Complex I in the mitochondrial electron transport chain and altered mitochondrial morphology. Loss of FLO18 function resulted in defective 5′‐end processing of mitochondrial nad5 transcripts encoding subunit 5 of nicotinamide adenine dinucleotide hydrogenase. These results suggested that FLO18 is involved in 5′‐end processing of nad5 messenger RNA and plays an important role in mitochondrial function and endosperm development.