Biological tissue is a complex medium, only anisotropic factor g is not enough to describe its scattering properties in many situations. The scattering properties of tissues can be characterized with parameters g and γ, where γ is a quantity that relates to second-order moment g2 of phase function, and shows the relationship between g2 and first-order moment g1. Some phase functions used to characterize the tissues at present and their second order parameter γ are studied, and the influence of phase function choice on γ measuement is discussed. The results demonstrate that γ is a measurable tissue quantity. These researches have teoretical significame and are practically valuable to consummate diffuse scattering theory and measure high-order parameter γ of tissues and other optical parameters.