A multiplex polymerase chain reaction (PCR) method was developed to simultaneously detect 4 varieties of genetically modified (GM) cotton. The event-specific primers were used to distinguish the 4 varieties of GM cotton (MON1445, MON15985, MON88913, and LLcotton25) using multiplex PCR. The acyl carrier protein 1 (Acp1) gene was used as an endogenous reference gene of cotton in the PCR detection. The primer pair Acp1-AF/AR containing a 99 bp amplicon was used to amplify the Acp1 gene and no amplified product was observed in any of the 13 different plants used as templates. This multiplex PCR method allowed for the detection of event-specific targets in a genomic DNA mixture of up to 1% GM cotton containing MON1445, MON15985, MON88913, and LLcotton25.