Arrays of carbon nanotubes (CNT) and diamond-clad Si tips were grown by microwave plasma-enhanced chemical vapor deposition. The former ones were grown directly on prepatterned cobalt-coated silicon substrate, while the latter ones were grown on Si-tip arrays. Each array contains 50 x 50 emitting cells and each individual cell is 3 mu m square. A maximum effective emission current density of about 17 A/cm(2) (at a macroscopic field of 17.5 V/mu m) has been demonstrated, while a macroscopic emission current density of 10 mA/cm(2) with operating fields around 10 V/mu m can be routinely achieved from an an ay of CNT emitters. In contrast, operating fields above 20 V/mu m were needed to draw a comparable emission current density from all of the diamond-clad Si tips arrays. Emission stability test performed at 40 mA/cm(2) for CNT arrays also showed little sign of degradation. Due to the high efficiency of electron emission, simple sample process, and large area growth capability, field emitter arrays based on CNT are attractive for flat panel display applications.