In order to investigate of Marigold (Tagetes erecta) allelopathic effects on insect diversity and diseasesof tomato (Solanum lycopersicum), an experiment was conducted as a split plot based on complete randomized block design with 3 replications at the Agricultural Research Station, Ferdowsi University of Mashhad, Iran, during 2010 and 2011. Treatments included three tomato varieties (Jina, Flat and strain hitack tork) and five cropping patterns (tomato monoculture and marigold-tomato intercropping) with ratios of 1:1, 1:2, 1:3 and 1:4. Results indicated that the highest percentage of predator populations was observed in tomato and marigold intercropping at a 1:2 ratio. The lowest percentage of predator populations was obtained in tomato and marigold intercropping at a 1:1 ratio. The highest Margalof and Simpson diversity indices of insect was obtained in marigold and tomato intercropping. In total harvestings, the highest relative frequency of healthy fruits was in tomato and marigold intercropping at a 1:2 ratio. Amongtomatovarieties, Hitak-strain Tork and Jina had the highest relative frequency of healthy fruits. The highest relative frequency of healthy fruits and the lowest relative frequency of unhealthy fruits were observed in the Jina variety (0.28 and 0.63 respectively). The positive significant regression relationship was obtained between Shannon and Margalof index with the relative yield of healthy fruits per unit area.