  We report temperature dependent Andreev reflection measurements of Co/
Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ (YBCO) heterostructure samples with
junction areas of 1 $\mu$m diameter. Modelling of the 5-70 K conductivity data
according to a modified Blonder-Tinkham-Klapwijk theory yields a spin
polarization in Co film amounting to 34% which is almost constant up to 70 K.
The YBCO films have been grown by pulsed laser deposition on sapphire
substrates. The Co films are deposited by thermal evaporation on YBCO. The film
is characterized by powder X-ray diffraction measurements which shows YBCO is
grown in (001) direction.The critical current density, 5 x 10$^{6}$ A/cm$^{2}$,
in YBCO remains nearly constant after deposition of Co at zero field and 77 K.

Superconductivity