Large scale (~0.6 sq deg) mapping of the giant Galactic star forming complex W 51 has been carried out simultaneously in two trans-IRAS far infrared (FIR) bands centered at ~200 and 130 μm using the TIFR 100 cm balloon borne FIR telescope. The choice of the bands is motivated by the aim of studying the emission from the cold dust (T< 30 K) component. These maps with ~1' spatial resolution, show extended emission with structures. Nearly identical beams in the two bands have enabled us to reliably determine the distribution of dust temperature T(130/200), and optical depth τ200, in the central region of the W 51 complex. A total of 25 discrete sources have been extracted from the 200 μm map, of which 8 have been associated with IRAS PSC sources and 16 with sources in HIRES processed IRAS maps at 100, 60, 25 & 12 μm bands. The values of the emissivity power law index β in FIR, show wide variation among these sources.