Dynamic behavior of plasma in internal modification of glass using fs-laser pulses with high pulse repetition rates up to 1MHz is observed using high-speed video camera at 50,000 frames per second. High-speed pictures reveal striking fact that the localized plasma is produced with size much smaller than reported in literatures before, and changes its location, size, shape and brightness periodically at repetition rates of the order of kHz. The mechanism responsible for producing localized plasma and its long-term periodic motion is presented based on the analysis of free -electron density based on rate equation. (C) 2014 Published by Elsevier B.V.