The aim of the study was: to evaluate the histomorphological alterations in canine mast cell tumors and the modification between these alterations; to assess the influence of histomorphological alterations in the clinical evolution of dogs with mast cell tumors; to evaluate the epidemiological and clinical characteristics of the tumor and verify if they are related to the death of the animal. Histopathological slides of mast cell tumors were analyzed for tumor extension, architecture, necrosis, eosinophilic infiltrate, margins, binucleated cells, mitotic index. These data were compared to the survival of the animals. Tumor architecture correlated with histological grade (HG) according to Kiupel (p=0,000002), mitoses was relationship with HG according to Patnaik grading system (p=0.003). Grade II tumors were more frequently observed in mixed-breed dogs (p=0.045), and animal death was related to presence of tumor ulceration (p=0.006) and age (p=0.001). In the correlation matrix, as association was observed between the number of eosinophils and tumor extension (0.221) and architecture (0.338); number of mitoses and architecture (0.339); Patnaik grading system and bleeding (0.190). In the log-rank test, architecture (p=0.006), number of mitoses (p=0.000), HG according Patnaik (p=0.045), and HG according Kiupel (p=0.000) influenced the survival of the animals. It was concluded that some histomorphological alterations, such as architecture, histological grade, number of mitoses and ulceration, can be used as prognostic factors in MCT in dogs.