Defects of the translation apparatus in human mitochondria are known to cause disease, yet details of how protein synthesis is regulated in this organelle remain to be unveiled. Here, we characterize a novel human protein, C7orf30 that contributes critically to mitochondrial translation and specifically associates with the large subunit of the mitochondrial ribosome (mt-LSU). Inactivation of C7orf30 in human cells by RNA interference results in respiratory incompetence owing to reduced mitochondrial translation rates without any appreciable effects on the steady-state levels of mitochondrial mRNAs and rRNAs. Ineffective translation in C7orf30-depleted cells or cells overexpressing a dominant-negative mutant of the protein results from aberrant assembly of mt-LSU and consequently reduced formation of the monosome. These findings lead us to propose that C7orf30 is a human assembly and/or stability factor involved in the biogenesis of the large subunit of the mitochondrial ribosome.