The effects of nozzle hole diameter on combustion and exhaust emissions were investigated using a single-cylinder DI diesel engine. (1) Exhaust NOx and combustion period depend only on injection pressure and not on nozzle hole diameter, because with a small-hole-dia. nozzle the effect of reduced mean injection rate is canceled out by the effect of promoted mixture formation and combustion. (2) With decreasing injection pressure, smoke sharply increases with a large hole-dia. nozzle but does not increase with small hole diameter of less than 0.20 mm. The decrease of injection pressure reduces NOx emission. (3) The effect of suppressing the increase of smoke with a small-hole-dia. nozzle is considered to be caused mainly by the rapid temperature rise over about 1800°C in the fuel-rich region near the spray in flame. (4) With the conversion of a deep bowl combustion chamber to a shallow-dish one, smoke increases as injection pressure decreases, in spite of the use of a small-hole-dia. nozzle, due to the reduction of mixing effect.