Biogas can be utilized in dual-fuel engines because of higher thermal efficiency. Biogas is supplied from intake port and liquid diesel fuel is injected in the cylinder directly. The combustion starts from the autoignition of a mixture of vaporized liquid fuel, further igniting biogas and air mixture. The initial combustion occurs at multi points, leading to certain and stable ignition followed by turbulent combustion. At first, visualization of the dual-fuel combustion with micro pilot injection is presented. The effects of liquid fuel injected, biogas flow rate, load, carbon dioxide (CO2) ratio in biogas, exhaust gas recirculation (EGR), compression ratio, H2 addition, pre-heating and other parameters are reviewed based on the literatures. Next, an example of the combustion achieving higher output and thermal efficiency with micro pilot dual-fuel combustion is described, as well as exhaust emissions. After the premixed mixture is autoignited in the end-gas region in latter half of the combustion, pressure oscillation does not occur in some conditions and transition to abnormal knocking combustion is avoided. The effect of CO2 ratio on PREMIER combustion was investigated and it was found that with higher concentrations of CO2 it was easier to keep control of PREMIER combustion.