  Solar radio type III bursts are believed to be the most sensitive signature
of near-relativistic electron beam propagation in the corona. A solar radio
type IIIb-III pair burst with fine frequency structures, observed by the Low
Frequency Array (LOFAR) with high temporal ($\sim10$ ms) and spectral (12.5
kHz) resolutions at 30 - 80 MHz, is presented. The observations show that the
type III burst consists of many striae, which have a frequency scale of about
0.1 MHz in both the fundamental (plasma) and the harmonic (double plasma)
emission. We investigate the effects of background density fluctuations based
on the observation of striae structure to estimate the density perturbation in
solar corona. It is found that the spectral index of the density fluctuation
spectrum is about $-1.7$, and the characteristic spatial scale of the density
perturbation is around $700$ km. This spectral index is very close to a
Kolmogorov turbulence spectral index of $-5/3$, consistent with a turbulent
cascade. This fact indicates that the coronal turbulence may play the important
role of modulating the time structures of solar radio type III bursts, and the
fine structure of radio type III bursts could provide a useful and unique tool
to diagnose the turbulence in the solar corona.
