Objective To explore the effect of microsurgical treatment of Chiari malformation type Ⅰ by small bone window foramen magnum decompression with removal of the outer layer of the dura. Methods Between January 2010 to December 2013, 27 patients with Chiari malformation type Ⅰ, 21 of whom with syringomyelia, underwent microsurgical treatment by small bone window foramen magnum decompression with removal of the outer layer of the dura. The suboccipital midline incision was approx 3.0～3.5 cm. The decompression bone window was approx 2.5 cm×2.5 cm. After the atlanto-occipital fascia was cleared thoroughly,the outer layer of the dura was longitudinally cut and expanded. Results There were no complications observed in this study. According Tator evaluation criteria, 20 cases of surgical treatment was excellentwith significant improvement in symptoms. Seven patients were determined as good for theri stable symptom without deterioration. Conslusion Microsurgical treatment of Chiari malformation type Ⅰ by small bone window foramen magnum decompression with removal of the outer layer of the dura is safe, effective, minimally invasive and easy to promote in the population.