  Recent observation of 850 micron sub-mm polarization from T Tauri disks opens
up the possibility of studying magnetic field structure within protostellar
disks. The degree of polarization is around 3 % and the direction of
polarization is perpendicular to the disk. Since thermal emission from dust
grains dominates the spectral energy distribution at the sub-mm/FIR regime,
dust grains are thought to be the cause of the polarization. We discuss grain
alignment by radiation and we explore the efficiency of dust alignment in T
Tauri disks. Calculations show that dust grains located far away from the
Central proto-star are more efficiently aligned. In the presence of a regular
magnetic field, the aligned grains produce polarized emission in sub-mm/FIR
wavelengths. The direction of polarization is perpendicular to the local
magnetic field direction. When we use a recent T Tauri disk model and take a
Mathis-Rumpl-Nordsieck-type distribution with maximum grain size of 500-1000
$\mu$m, the degree of polarization is around 2-3 % level at wavelengths larger
than $\sim100\mu$m. Our study indicates that multifrequency infrared
polarimetric studies of protostellar disks can provide good insights into the
details of their magnetic structure. We also provide predictions for polarized
emission for disks viewed at different wavelengths and viewing angles.
