The Sendai virus (SeV) C proteins are a nested set of four accessory proteins, C', C, Y1, and Y2, encoded on the P mRNA from an alternate reading frame. The C proteins are multifunctional proteins involved in viral pathogenesis, inhibition of viral RNA synthesis, counteracting the innate immune response of the host cell, inhibition of virus-induced apoptosis, and facilitating virus-like particle (VLP)/virus budding. Among these functions, the roles for pathogenesis and counteracting host cell interferon (IFN) responses have been studied extensively, but the others are less well understood. In this paper, we found that the C proteins contributed in many ways to the efficient production of infectious virus particles by using a series of SeV recombinants without one or more C protein expression. Knockout of both C' and C protein expression resulted in reduced virus release despite higher viral protein synthesis in the cells. Interestingly, for the viruses without C' and C, or all four C protein expression, non-infectious virions containing antigenomic RNAs were produced predominantly compared to genomic RNA-containing infectious virions, due to aberrant viral RNA synthesis. Our results demonstrate for the first time that the C proteins regulate balance of viral genome and antigenome RNA synthesis for efficient production of infectious virus particles in the course of virus infection.