Electron beam (EB) lithography has often been used for fabricating advanced ULSIs. Recently, to increase the writing throughput, electron beam projection lithography (EBPL) technology has been proposed (100kV acceleration voltage and 20-30(mu) A total currents). When we introduce the EBPL to practical use, it is important to develop a projection mask and a mask data conversion system, because there are many differences between the EBPL mask data and the conventional EB data. In EBPL mask conversion system, it is necessary to divide a full chip data into several 1mm X 1mm (250micrometers X 250micrometers on the wafer) sub-fields, which size is as same as one EBPL shot with format conversion. In this paper, we show the data conversion system that converts pattern data (GDS+U) to EBPL mask data. This system can maintain the hierarchy data structure in the dividing process. The patterns that located on the boundary between neighboring fields were treated as belonging in either field for preventing critical division. As a result, a data conversion system for EBPL mask, that can divide the device data with high speed and high quality, was successfully obtained.