We present small-sized depletion-type silicon Mach-Zehnder (MZ) modulator with a vertically dipped PN depletion junction (VDJ) phase shifter based on a CMOS compatible process. The fabricated device with a 100 μm long VDJ phase shifter shows a VπLπ of ∼0.6 V·cm with a 3 dB bandwidth of ∼50 GHz at -2 V bias. The measured extinction ratios are 6 and 5.3 dB for 40 and 50 Gb/s operation under 2.5 Vpp differential drive, respectively. On-chip insertion loss is 3 dB for the maximum optical transmission. This includes the phase-shifter loss of 1.88 dB/100 μm, resulting mostly from the extra optical propagation loss through the polysilicon-plug structure for electrical contact, which can be readily minimized by utilizing finer-scaled lithography nodes. The experimental result indicates that a compact depletion-type MZ modulator based on the VDJ scheme can be a potential candidate for future chip-level integration.