Diffusion-weighted imaging (DWI) is an imaging manner that can detect water motion within intra, extra, and transcellular areas. Also, it can distinguish brain tumors either benign or malignant. Estimating the importance of diffusion MRI in differentiation between neoplastic and non-neoplastic brain lesions. The study was executed in the Radiology department at Al-Azhar University Hospitals of Damietta and included 40 patients of both sexes. The procedure was done using a Philips Achiva 1.5-Tesla unit system scanner. All the patients had undergone conventional MRI examination including axial, sagittal, coronal T1, axial and coronal T2 and axial flair. Post-contrast administration is axial, coronal and sagittal T1 weighted images. DWI was performed and the ADC map was generated by MRI software. We determined the signal intensity of the lesion. ADC was measured in the solid part of the lesion. Finally, the findings were correlated with the histopathological findings. Apparent diffusion coefficient (ADC) was significantly lower in high-grade malignant tumours than that of low grades, while low-grade glioma has significantly higher ADC than high-grade one, there was significantly lower ADC in patients with lymphoma than those with high-grade astrocytoma, and significant higher ADC for necrotic tumours than brain abscess. DWI is a highly sensitive and non-invasive technique that helps in the differentiation of different brain lesions. DWI must be used together with conventional MRI.