A process for manufacturing monochloroacetaldehyde trimer and chloral together by effectively utilizing a raw material acetaldehyde or para-aldehyde. The process comprises a step of chlorinating acetaldehyde or para-aldehyde to produce a chlorinated liquid of which the major component is monochloroacetaldehyde, a step comprising adding chloral to said chlorinated liquid and distilling the mixture to obtain a fraction of which the major components are monochloroacetaldehyde and chloral, a step of trimerizing monochloroacetaldehyde by reacting said fraction in the presence of a trimerization catalyst and separating the MCA trimer by filtration, and a step of chlorinating other fractions from said distillation step and the filtrate from said trimerization step to produce chloral. According to this process all raw material aldehydes and components derived from aldehydes which have not been consumed for the production of MCA trimer can be easily converted into chloral which is useful as an industrial chemical. The process is also free from the problem of the waste water treatment.