Abstract. Dispersion Measure (DM) is one of the most important factors which can limit the accuracy with which one is able to time pulsars. The DM of a pulsar, in general, changes significantly with time due to reasons such as proper motion of the pulsar, plasma density changes in the binary orbit, drift of intervening inhomogeneous ISM, solar wind plasma. Hence, it becomes necessary to keep track of the changing DM, especially for pulsar timing applications requiring very high timing precision, such as the various pulsar timing array projects which aim to detect gravitational waves. Drawing motivation from this as well as from an earlier work by Ahuja et al. (2005), who had achieved a DM accuracy of 1 part in 104 for long period pulsars, we initiated a project aimed to do regular simultaneous multi-frequency timing observation of a selected set of millisecond pulsars using the GMRT with the aim of studying (i) accurate DM variations of MSPs and their effects on timing accuracy and (ii) DM variations due to the solar corona and the solar wind. Preliminary results from the observations from previous three cycles are already demonstrating accuracies of DM measurements of a few parts in 105 for the MSP observations. With the further enhancements in observing modes of the GMRT software back-end that are planned, coupled with improved analysis techniques, it will soon be possible to get to our target of 1 part in 105 or better.