Using a data sample of ψ(3770) events collected with the BESIII detector at BEPCII corresponding to an integrated luminosity of 2.9 fb-1, we report a measurement of Λ spin polarization in e+e-→ΛΛ¯ at s=3.773 GeV. The significance of polarization is found to be 2σ including the systematic uncertainty, which implies a zero phase between the transition amplitudes of the ΛΛ¯ helicity states. This phase can be interpreted in terms of psionic form factors, and is determined to be ΔΦΨ=ΦEΨ-ΦMΨ=(71-46+66±5)°. Similarly, the ratio between the form factors is found to be Rψ=|GEΨ/GMΨ|=0.48-0.35+0.21±0.03. The first uncertainties are statistical and the second systematic.