Events of the Time of Troubles of the early 17th century played a great role in the history of Russia. In the year of 1609, the Polish royal army under high command of hetman Stanislav Zholkevsky and the King Sigismundus III himself invaded Russia and resiged Smolensk. At this time, a massive offensive to Western, South-Western and North-Western regions of Russia was launched by some detachments under Aleksander Korvinus Gosievsky, some of Polish colonels who were at the head of their regiments and the Cossacks of Zaporozhye. The role of this invasion was very important. The Polish authorities viewed the Russians as a potentially oppositional force, though actual economic collapse and repressions from the state authorities of Vasily Shuiski and the false Dimitry II considerably undermined their enthusiasm and brought social apathy among the Russians. Nevertheless, a garrison of Smolensk refused to surrender and decided to fight against the enemy. History of this siege was described by the King’s secretaries in King Sigismundus III’s Diary about Campaign to Russia. It belongs to the main sources about Russia’s Time of Troubles and about these events, as well as other documents of foreign observers. Using the procedures developed by R.G. Skrynnikov and I.O. Tyumentsev, the authors found and analysed unknown fragment of the “King Sigismundus‘ journal”, which are edited in this publication for the first time. Comparison of this document with the another data of King Sigismundus III’s Diary about Campaign to Russia and other Polish-Luthuanian sources from the beginning of the 17th century enables them to carry out a complex examination of the materials and to cross-verify their data. Experience shows that this technique works effectively.