The main target of this study was to reveal the relationship between incidences of each of CRD andIB disease in broilers, clinically showing signs of severe respiratory disorders from production fields at Nineveh province, as well as highlighting the relations between those two infections and the occurrence of lesions in lower respiratory tract For serving the purpose, 198 broiler bird was isolated after confirmed showing severe respiratory disorders from 25 production field in Nineveh province for the period from 1/11/2021 to 1/2/2022. The clinical observations were recorded, blood serum tested by ELISA to confirm infection with Mycoplasma gallisepticum and IB virus antigens, recording the gross pathological notes and for harvesting samples of the lower respiratory system which preserved, then 99 samples of them were randomly selected for histopathological examination that studied lesions in pulmonary parenchyma. The results demonstrate a high variation in incidence rate of the infection between the CRD by Mycoplasma gallisepticum reaching 81.3%, were the incidence rate of infection with IB was 1.5 %, the incidence rate of the mixed infection was 1.5% as well. Gross examination also supported the clinical signs by revealing pathological changes with the inflammatory nature at all the examined birds including pulmonary congestion, fibrin deposition, adhesions between pulmonary lobs and air sacs and cheesy deposition in bronchi and airs sacs. There was hyperemia and pulmonary edema at ratios 91.91% and 84.84% respectively, pleuropneumonia and pleural fibrosis and thickening at ratios 32.32% and 19.19% at pulmonary-pleural junction area. These lesions were variated in their severity between samples The correlation test results showed a significant link between infection with Mycoplasmosis and hyperemia, pulmonary emphysema at (P≤0.01) also there was a significant correlation between Mycoplasma infection and each of perivascular hemorrhage, cellular inflammatory infiltrations, caseous necrosis and pulmonary fibrosis at (P≤0.05) as well as there was a significant correlation link (P≤0.01) between IB and pleural fibrosis. The study concluded that both of these two infections were related to incidence of lesions but they more synchronized and correlated to the infection of Mycoplasma gallisepticum while very limited pathological relevance were linked with IB, may be due to the success of the applied vaccination programs to control it in fields.