To enhance the sensitivity of an ELISA developed for the acaricide/insecticide fenazaquin, the length of the spacer arm attached to the 7-position of the quinazoline ring in the hapten (H-I) used previously as an immunogen was shortened, and this modified hapten (H-IV) was then used as an immunogen. The new immunogen used to raise polyclonal antibodies was designated 3-(4-(4-tert-butylbenzyloxy)quinazolin-7-ylcarbamoyl)propanoic acid. Under optimized conditions of a competitive indirect ELISA (ciELISA), the former $IC_{50}$ value of 96.97 ng/mL was reduced to 42.13 ng/mL, and the detection range was 10.96-161.91 ng/mL, the lowest detection limit being 1.8 ng/mL. HPLC was run to compare with and to validate the ELISA with fortified fruit and water samples; the correlation was 0.997-1.000 (n=4-5) with a slope of 0.905-1.112. The recoveries of fenazaquin from fortified samples by the ELISA ranged from 90.61 to 115.12%, which is acceptable as an ELISA for residue analysis.