Although Thailand has previously applied the Sufficiency Economy Philosophy in its development paradigm, few quantitative studies focus on the magnitude of the effect of Sufficiency Economy Philosophy on happiness. This focuses on the farmers who live in upper northern Thailand. Result from Ordered Logit regression shows that Sufficiency Economy Intensity (SEI) has a strong positive relationship with subjective happiness. The more the SEI adaptation, the happier the farmers are. Subjective happiness is also positively correlated with objective happiness and absolute income. Comparing the effect of all including variables, SEI is the most powerful variable to increase happiness while the power of objective happiness and absolute household income are limited. While real relative income variables are insignificant, attitude toward relative income has a strong positive significant relationship with the subjective happiness level. That means “attitude or perception” is more important than “real situation” in the view of farmers. Moreover, farmers with less income aspiration or ambition seem to be happier than those with more. After the effects of objective happiness, household income, relative income, and income aspiration were controlled through the research model, SEI still affected subjective happiness. This result confirms that Sufficiency Economy Philosophy can effectively affect one’s mental happiness, which is the core concept of the Sufficiency Economy Philosophy.