A planetary gear train of an automatic transmission is provided to improve power delivery and fuel efficiency by applying six engagement elements to four planetary gear sets to achieve multiple speeds. The planetary gear train includes: input and output shafts; first to fourth planetary gear sets respectively including first to third rotation elements, fourth to sixth rotation elements, seventh to ninth rotation elements, tenth to twelfth rotation elements; and first to eighth shafts. In particular, the first shaft is fixedly connected to the second, sixth and eleventh rotation elements and the input shaft, the fourth shaft is fixedly connected to the fourth, seventh and tenth rotation elements, and the seventh and eighth shafts selectively and respectively connect a transmission housing to corresponding rotation elements of the first and third planetary gear sets, and the corresponding rotation elements are not connected to any shaft among the first to sixth shafts.