This musical analysis is mainly to explore the musical form of the piece, Piano Sonata in Bb major, K. 333, first movement by Wolfgang Amadeus Mozart. The purpose of this analysis is to reveal the design and harmonic organization of this piece. Besides that, the techniques that contribute to the continuous unbroken flow of musical ideas in the piece are examined. Finally, the interplay between tension and relaxation happened in the piece is discussed too. Score analysis and observation technique are applied in this study. The phrase structural organization is fundamental in this analysis. Examination of musical elements like harmony and melody is done from phrase to phrase. It is discovered that the technique of opposing ideas, repetition, sequence, and motivic development, and motivic variation have contributed to the flow of musical ideas in this composition. The thematic relationship and motivic relationship between segments has shown the continuity and unity within the whole composition.