Variability is one of the most important features of pre-main-sequence (PMS) stars. Several mechanisms are known to induce this variability, for example, irregular distribution of cool spots on the stellar photosphere, variable hot spots, obscuration from dust, instability in disk, change in accretion rate, etc. Time variable accretion and dust extinction play a prominent role in the non-periodic variability, whereas the presence of cool and/or hot spots on the photosphere of the rotating star produce periodic/quasi-periodic changes in their light curve. The period of a rotating star is a direct indicator of its rotation period and hence related to angular momentum which is one of the most important fundamental parameters of a star. The role of the disk-locking mechanism in angular momentum removal from the young star is presently under debate. In this thesis, I have studied the variability properties of young stars using the V, R, and I optical bands in three young open clusters, Sh 2-170, Sh 2-190, and IC 1848 to understand the relation between period and amplitude with the stellar parameters (age, mass, infrared excess) which further gives us insight into the angular momentum evolution of young stars and the role of the disk in their variability. Typically the period of these variables ranges from a few hours to a few weeks while the amplitude varies up to a few mags in the optical bands. Class II and Class III objects show different periodic behavior and also the former shows a larger amplitude of variation, in general. We will discuss these properties of PMS variable stars.