Alternaria alternata one of widely distribution plant pathogenic and saprophytic fungi. A. alternata producing more than 70 secondary metabolites, One of the important metabolites (AOH). The main effect of alternariol (AOH) lies in the generation reactive oxygen species (ROS) in rat's liver cytochromes. The aim of this work was studying the detection of polyketide synthase gene that responsible for Alternariol (AOH) production from Alternaria alternata by specific primer. Detection of AOH production by thin layer chromatography and the PKSJ gene by specific primer in PCR thermal cycle.43 sample of infected tomato by early blight disease in Karbala city collecting and cultured on PDA. For detection of mycotoxins (AOH) production. Designed this primer through the use of the complete sequence of the PKSJ gene (Gene bank sequence JX103645.1) from the site of Gene bank-NCBI and Primer3plus using the program to design primers and used in the PCR test. The result showed 23 out off 24 isolates were produced AOH toxin. TLC plate (Silica gel G60 20x20cm) was used for detection of AOH in comparison with OTA standard spot. 23 isolates was produced AOH toxin. TLC plate was used for detection of AOH in comparison with OTA standard spot. PKSJ primer that designed in this study was success for the detection and investigation of the gene responsible for the production of AOH, where the primer could amplify the target piece of PKSJ gene and produce bands by molecular weight 514 bp on agros gel for all isolates that produced the toxin according to the TLC results. Most isolates of A.alternata can produce AOH toxin. The specific PKSJ primer was success by PCR amplified the target gene with all isolates excepted isolate No. 2 which not produce AOH, finally the PKSJ primer is specific primer for detection of polyketide synthase gene.