We present an experimental study of X-ray generation from nanostructured ZnO targets. Samples of different morphology ranging from nanowires to polished surfaces are irradiated by relativistically intense femtosecond laser pulses. X-ray emission of plasma is generated by 45 fs 130 mJ laser pulses at 400 nm with picosecond temporal contrast better than 10~(-9) interacting with an array of ZnO nanowires. The measured spectra indicate the existence of highly ionized states of Zn (up to He-like Zn). The obtained flux of ～10~(10) photons per laser shot at the neutral Zn K_α energies around 8.65 keV and at the Zn He_α energies around 9 keV is almost 3 times higher for nanostructured targets compared to the reference polished sample and implies 10~(-4) conversion efficiency from the laser energy to the total energy of the emitted X-ray photons.
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