From the view of suppressing the global warming and environmental pollution, responding to the regulation of fuel consumption and exhaust gases along with lengthening the maintenance interval, are becoming more demanded. The development of a high-performance, long-life spark plug has become essential in response to these demands. While improved performance (high ignitability and low required voltage), the discharge part of the spark plug needs to be reduced in size. But, in the past this has been difficult because of the limitations of platinum alloys in terms of wear. Therefore, it has been quite difficult to achieve both smaller discharge parts and longer life.To dramatically improve wear resistance, we researched materials that are both resistant to oxidation and have a high melting point. This research resulted in our development of a new iridium alloy (Iridium-10wt%Rhodium).Through this development we have been able to produce an iridium spark plug that surpasses the conventional platinum technology. We set the following two types of spark plugs that could be mass produced for first time in the world. 1) Ultra-high ignitability spark plugs (electrode Φ 0.4 mm and same service life as platinum spark plugs) 2) High ignitability spark plugs (electrode Φ 0.7 mm and twice the service life of platinum spark plugs)This paper particularly presents concrete engine evaluation results for the ultra-high ignitability spark plugs with the electrode Φ 0.4 mm and proposes examples of improving combustion by using these spark plugs. For example, improved ignitability makes possible reduced idling revolutions, larger EGR, leaner air-fuel mixtures, and emissions.