In this paper we study the effect of positional randomness on transmissional properties of a two dimensional photonic crystal as a function of a randomness parameter α (α = 0 completely ordered, α = 1 completely disordered). We use finite-difference time-domain (FDTD) method to solve the Maxwell's equations in such a medium numerically. We consider two situations: first a 90 • bent photonic crystal waveguide and second a centrally pulsed photonic crystal micro-cavity. We plot various figures for each case which characterize the effect of randomness quantitatively. More specifically, in the wave-guide situation, we show that the general shape of the normalized total output energy is a Gaussian function of randomness with wavelength-dependent width. For centrally pulsed PC, the output energy curves display extremum behavior both as a function of time as well as randomness. We explain these effects in terms of two distinct but simultaneous effects which emerge with increasing randomness, namely the creation of semilocalized modes and the shrinking (and eventual destruction) of the photonic band-gaps. Semi-localized (i.e. Anderson localized) modes are seen to arise as a synchronization of internal modes within a cluster of randomly positioned dielectric nano-particles. The general trend we observe shows a sharp change of behavior in the intermediate randomness regime (i.e. α ≈ 0.5) which we attribute to a similar behavior in the underlying overlap probability of nano-particles. PACS. 42.70.Qs Photonic bandgap materials -72.15.Rn Localization effects -73.20.Fz Weak or Anderson localization -78.67.Bf Optical properties of nano-crystals and nano-particles