The ⁷Li NMR Knight shift was measured for liquid Li-Tl alloys. Prior to this NMR measurement, DSC measurements were carried out to know the exact liquidus temperatures. Except for some trivial points, the phase diagram determined is almost same as the reported one; the solid compound at the maximum liquidus temperature was observed at 50 at% Tl; the eutectic point was detected at 84.8 at% Tl. The ⁷Li Knight shift, K, decreases rapidly with the addition of Tl up to 20 at% Tl. In the concentration range from 20 to 50 at% Tl, the K keeps almost constant value, which is 60% of ⁷Li Knight shift for the pure liquid Li. Such a decrease of the K is considered as an indication for the strong charge transfer from Li to Tl. The Zintl ion formation is expected in this concentration range of liquid Li-Tl alloys. These tendencies are similar to the previous studies for liquid Li-Ga and Li-In alloys. However, beyond 50 at% Tl, the K increases slightly and reaches to the constant value (70% of ⁷Li Knight shift for the pure liquid Li). Such a back donation of charge is absent for liquid Li-Ga and Li-In alloys. It is considered that the tendency of the Zintl ion formation for liquid Li-Tl alloys is slightly weaker compared with the cases of liquid Li-Ga and Li-In alloys.