Recent observations of 850 mum sub-mm polarization from T-Tauri disks open up the possibility of studying the 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 sub-mm/far-infrared (FIR) wavelengths, 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. The 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 mum, the degree of polarization is around 2-3% level at wavelengths larger than ∼ 100 mum. Our work 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.