Probability criterion is an investment decision-m ak ing criterion of investo rs in som e cases, it has p ract ical sign if icance, it s investm en t decision2m ak ing is determ ined by the expected retu rn rate. Generally speak ing, differen t investo rsw ill have differen t expected retu rn rates, the secu rity’s retu rn rates in differen t periods w ill have differen t p robab ility dist ribu t ion s and co rrelat ion to each o ther. In th is paper, a p rob lem of two2period investm en t decision2m ak ing w ith p robab ility criterion is p ropo sed, it s m athem at ical model of an op t im al investm en t decision2m ak ing is estab lished. Fo r con t inuou s o r discrete random variab le of secu rity’s retu rn rate, the step s fo r investm en t decision2 m ak ing are derived. F inally, an illu st rat ive examp le is given.