The non-breakdown and breakdown streamers in paraffinic model oil were investigated under negative step voltages. Experiments were performed with the point-plane gap of 8 cm and voltage up to 540 kV. Streamer behaviors were observed with shape, stopping length, velocity, current and light emission pulses. Carbon particles markedly reduce inception and breakdown voltages of streamers, while dissolved gases have a little effect on these voltages. Both low ionization potential and electron scavenging additives accelerate the streamers. However, the electron scavenger shows a much stronger influence on streamers than the low ionization potential additive. © 1994-2012 IEEE.