Context : Unevolved metal poor stars are the witness of the early evolution of the Galaxy .
The determination of their detailed chemical composition is an important tool to understand the chemical history of our Galaxy .
The study of their chemical composition can also be used to constrain the nucleosynthesis of the first generation of supernovae that enriched the interstellar medium .

Aims : The aim is to observe a sample of extremely metal poor stars ( EMP stars ) candidates selected from SDSS DR12 release and determine their chemical composition .

Methods : We obtained high resolution spectra of a sample of five stars using HDS on Subaru telescope and used standard 1D models to compute the abundances .
The stars we analysed have a metallicity [ Fe/H ] between -3.50 dex and -4.25 dex .

Results : We confirm that the five metal poor candidates selected from low resolution spectra are very metal poor .
We present , the discovery of a new ultra metal-poor star ( UMP star ) with a metallicity of [ Fe/H ] = -4.25 dex ( SDSS J1050032.34 - 241009.7 ) .
We measured in this star an upper limit of lithium ( log ( Li/H ) \leq 2.0 .
We found that the 4 most metal poor stars of our sample have a lower lithium abundance than the Spite plateau lithium value .
We obtain upper limits for carbon in the sample of stars .
None of them belong to the high carbon band .
We measured abundances of Mg and Ca in most of the stars and found three new \alpha -poor stars .

Conclusions :