By assimilating historical observation with NWP model output, reanalysis datasets provide long-term comprehensive description of the history of the climate system in recent decades. They are the base of a large amount of current knowledge about the climate trend. On the other hand, the inhomogeneity in reanalyses, which stem from changes in the observing system and biases of model, causes concerns about the reliability of the trends derived from reanalyses. In this presentation, first we will discuss about the discontinuities in previous reanalysis datasets and the associated uncertainties in climate change trend study. Then we will present the conception and some preliminary results of an ongoing effort to homogenize the NASA MERRA reanalysis dataset.