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• The used big grain PCD films allow the effect of grain boundaries to be ignored. • Compact needles with high aspect ratio were obtained at a rate of 2 μm/min. • O 2 plasma etching generated trans -polyacetylene was suppressed by Cl or H ions. • Preferential etching and needles formation were investigated time-dependently. The needle-like surface morphology evolution in oxygen plasma in combination with a secondary gas (Cl 2 , CHF 3 or CF 4) by inductively coupled plasma reactive ion etching (ICP-RIE) on a free-standing polycrystalline diamond was investigated. The addition of CF 4 can produce trans -polyacetylene (t -PA), which is similar to the result when the pure O 2 etching takes place, and generate compact needle-tip particles. However, the t -PA disappears with the introduction of Cl or H ions. The optimised etching parameters for the needle-like structure formation are as following: Cl 2 /O 2 ratio 20% and RF-power (RFP) 100 W, where more compact and even nano-needles are realised with an average etching rate of 2 μm/min. The Cl 2 /O 2 plasma etching results indicate that the time-dependent etching mechanism of diamond nano-needles results from (1 1 1) crystal plane selective etching and preferential graphitisation at the twin-plane boundary and dislocation area. [ABSTRACT FROM AUTHOR]