Seyyed Hasan Taqizaade (1878-1970) played an important part in political, diplomatic, and cultural fields in the 20th century's Iran. He was active as a nationalist and constitutionalist during and immediately after the Iranian Constitutional Revolution (1906-1911). In this article, we discuss his nationalism and constitutional thought by analyzing his two writings, firstly, Tahqiq-e Haalaat-e Konunia-ye Iaraan baa Mohaakemaat-e Taariakhia (An Analysis of the Present Situation of Iran through Historical Judgement) , an article written in 1904-05, immediately before the Revolution, and secondly, his articles in the Persian newspaper Kaave, written from 1916 to 1921 in Berlin. In the former article, Taqizaade evaluates Iranian history from the ancient times to the contemporary era with an 'objective' stance based on historical evidences. He appreciates ancient Iran to be prosperous, but does not overestimate it, attributing the main cause of its decline to the internal corruption of rulers, which resulted in the Arab conquest. According to him, the Arabic conquest was a cause of Iran's stagnation, but the Mongolian conquest caused a more disastrous effect, after which a disparity in the level of civilization between Iran and the West began to expand. However, the Qaajaar dynasty has been unaware of it, continuing internal despotism. Taqiazaade makes two proposals to reform this corrupted situation, the first, the introduction of Western constitutional system, and the second, the introduction of Western sciences and educational system. On the whole, compared to the extreme nationalistic tendency at that time admiring the ancient history and traditions of Iran without any criticism, he seemed to be a relatively moderate nationalist. In the articles of Kaave, Taqizaade takes even more critical stance towards the sociopolitical situation of Iran during and after the Revolution. He criticizes the new elite class who participated in the Revolution with enthusiastic patriotism, but gradually became extreme nationalists or mere politicians. At the same time, he is very critical of the ignorance of the masses, who have been easily influenced with the extreme patriotic propaganda, or begun the shallow imitation of politicians in the excessively political trend after the Revolution. Also, the moral corruption of a part of 'ulama (clergy) with an extreme inclination to the Islamic law is criticized. According to him, these problems are the obstacles to found a complete constitutional system in Iran. This indication is quite significant in order to reassess the role, in particular, of the masses in the Revolution, which seems to have been often 'negative' from his point of view. Taqizaade suggests these problems stem from a lack of the exact comprehension of Western sciences and political system. Therefore, he advocates educational reform with the primary purpose of the expansion of a literacy rate among the masses. In comparison with the former article, Taqiazaade stresses the importance of educational reform instead of political reforms in Kaiave. However, we can see in his two writings the same belief in establishing an modern nation-state on the basis of the moderate nationalism.