In this paper, an investigation of the efficiency of fully on-chip DC-DC step-up converter realized in a standard 130 nm CMOS technology is presented. The converter has been designed for regulated output voltage of 1.2 V with an on-chip inductor and an output capacitor. The obtained results show that the proposed on-chip converter can achieve the efficiency up to 98 % and the output power up to 6 mW. The performed analysis investigates possibilities for the further improvement focused on the maximum efficiency and output power.