This paper is an attempt to discuss how the crisis after the defeat of Athenian fleet in Sicily (413 B.C.) contributed to the Athenian constitutional changes in 411-410 B.C.. In this attempt, the author tried to throw some new light on the much discussed problem about the Athenian revolutions of 411 B.C.. The problem is whether the constitution of the Five Thousand denied the franchise to those who were below the hoplite class or not. At first, the author divided Athenian citizens into two classes. A man who could be a hoplite belongs to the one class, and one who could not to the other. And then, to inquire into political trends of those two classes, he tried to apply two models which W.R.Connor proposed. This inquiry leads the author to the conclusion that the accepted view (that the regime of the Five Thousand was a hoplite oligarchy) is acceptable. His arguments are as follows. 1)After the Sicilian disaster, there was a change in the balance of the political powers in Athens. Citizens belonging to the lower class were depressed and their influence was revived only when the crushing victory was gained at Kyzikos in the summer of 410 B.C., as C.Hignett inferred. 2)There is no reason to think that the upper class citizens' in Athens were against the radical democracy in any situation. As Prof. Connor told us, there were many small political groups among them. Their attitude as a whole to the Athenian constitution is changeable according to circumstances. The author inferred that until the Council of Four Hundred was established, the major political goal among the political groups in the upper class citzens was to replace the radical democracy by the oligarchy, that in course of transition from the regime of Four Hundred to that of Five Thousand, the political goal changed and the new goal was to carry on their war against Sparta more effectively, and that on the day of the restoration of the radical democracy, many political groups wanted Alcibiades not as a tyrant of Athens. Under the regime of Four Hundred, the war situation grew even worse. It caused the transition to the regime of Five Thousand. The radical democracy was restored not because the constitution of the Five Thousand allowed the lower class the right of voting (against G.E.M. de Ste. Croix and R.Sealey), but because the upper class citizens feared that Alcibiades would be a tyrant. 3)Constitutional changes in Athens, changes in the balance between the forces for Alcibiades and against, and those in the tide of war against Sparta, all three ran in parallel with one another. Alcibiades was a brilliant general but a would-be tyrant. Through the participation of Alcibiades in Athenian affairs, the crisis after the Sicilian disaster contributed seriously to the Athenian constitutional changes in 411-410 B.C..