Abstract. With the launch of Fermi in June 2008, which has the sensitivity to measure the emission from bright TeV blazars on daily to weekly time scales, it is now possible to put together high quality data to build simultaneous and well sampled spectral energy distribution of blazars in their flaring state. With the aim of obtaining a broad spectral energy distribution of Mrk-421 during its high state, a detailed analysis of the Fermi-LAT and RXTE/ASM data recorded between 12th Feb. 2010 to 25th Feb 2010 has been performed to study the daily flux/spectral variability of the source. The analysis involves studying the daily flux/spectral variability of the source in 100 MeV - 100 GeV and 1 GeV - 100 GeV energy bands by using the Fermi-LAT data. The light curves of the source in the above mentioned energy bands reveal flux variability by a factor of ∼2 with TS > 25 (i.e ∼ 5σ detection) on several nights with peak flux values of ∼5 x 10^-7 cm^-2 S^-2 and ∼1 x 10^-7 cm^-2 s^-1 in the above two mentioned energy bands. Apart from presenting the light curves of the source as recorded by the Fermi-LAT in 100 MeV-100 GeV energy band and by the RXTE/ASM in X-rays, we will also discuss aspects like correlation coefficient and variability index in the paper.