This paper examines the change of frequency of the core modal auxiliaries will/would, shall/should, can/could, may/might, and must in the Corpus of Historical American English (COHA) covering 1810 to 2009. The most common modals used in this period were would (average 2,827.69 per mil), followed by will (2,262.94), can (1,783.77), could (1,782.83), may (1,138.85), should (1,081.55), must (1,019.18), might (729.83) and shall (609.54). The top four modals accounts for 65.4%. The general trend is a significant decline in use of core modals with time: In a simple comparison between 1810s and 2000s, shall reveals the most remarkable decline (-97.23%), then follows must (-78.34%), may (-77.76%), will (-65.48%), should (-59.21%), can (-29.53%), might (-27.17%), would (-17.13%), though can and would shows repetition of rises and falls, and could shows a rise in frequency. This paper shows that the decline of core modals goes along with the increase of such lexical verbs as semi-auxiliaries and main verbs, and argues for grammaticalization and democratization as the determinants of this change: The increase of lexical verbs is due to a 'renewal' in a grammaticalization process and the notable loss of must and shall is related to the sociocultural change of democratization.