Molecular beam expitaxial (MBE) grown GaAs at low substrate temperature (LT-GaAs) possesses a unique combination of properties (i.e., semi-insulation and short carrier lifetime) that has led to a variety of electronic and photonic device applications. In this paper, we report on the optical characterization of LT-GaAs, including carrier lifetime, photoreflectance (PR), and surface photovoltage (SPV) measurements. The undoped LT-GaAs samples were grown using our ow custom designed MBE system at the following substrate temperatures: 200 degree(s)C, 250 degree(s)C and 300 degree(s)C. These sample were then annealed at 7000-850 degree(s)C in a rapid thermal annealing (RTA) system. The PR spectra revealed that the PR amplitude depends strongly on the carrier lifetime, while the PR spectral broadening of near bandgap peak depends strongly on the internal field non-uniformly caused by buried Schottky barriers around the As precipitates. Above bandgap SPV measurements revealed a unique SPV spectrum compared with that for bulk GaAs. Carrier lifetime was measured for LT-GaAs samples grown at 200, 250, and 300 degree(s)C, respectively, and annealed at 700 degree(s)C for 30 seconds, and the corresponding carrier lifetimes at 1.5, 2.2, and 12 ps.