In internal combustion engines the engine vibrations come up as block vibrations and crankshaft speed fluctuations. Crankshaft speed fluctuations indicate the unstable running of the engine. Crankshaft speed fluctuations are caused by several sources. Main factor affecting the crankshaft speed fluctuations is in-cylinder pressure. Changes in-cylinder pressure caused by cyclic variations and misfire result as speed fluctuations on a crankshaft. In this study the effects of the cyclic variations and misfire problem on the engine crankshaft speed fluctuations have been investigated. A mathematical model including engine kinematics has been developed for a four cylinder diesel engine. In-cylinder pressure profiles used in the mathematic model were obtained experimentally. Two pressure profiles including 11 cycles and averages of these cycles were used in analysis. Pressure profiles were expressed mathematically by Fourier series having 1001 term. Although the indicated mean effective pressure values of pressure profiles were stable, the crankshaft speed fluctuations were determined as 5.5% and 11.1% at 230 rad/s for 5.15% and 12.92% COVimep values. When single misfire take place in the third cylinder, average crankshaft speed decreased 6.6 rad/s. Also, in case of continuous misfire the crankshaft speed fluctuations increased from 4.3% to 8%.