At a certain crank angle just after opening or just before closing of poppet valves (suction or exhaust) in the internal combustion engine, there is practically little suction or exhaust-action, so that such a crank angle is termed an ineffective angle and the corresponding valve lift is called an ineffective lift. To examine the effects of engine speed and pressure difference between suction and exhaust sides on the ineffective angle or lift, we carried out some experiments by an air-cooled single-cylinder four-stroke engine. Certain results obtained are summarized as follows : a) Regardless of engine speed and pressure difference, the minimum value of the ineffective angle remains constant at about 13°∼15° i.e. the ineffective valve lift is equal to about 32/100 mm in our experiments. b) The ineffective lift increases in proportion to the engine speed. c) On the other hand, the lift decreases with increasing of the pressure difference, particularly it contracts to the value of 32/100 mm at the pressure difference above 500 mm Aq.