The purpose of this research was to describe the effect of fully online learning to the mathematical reasoning abilities of students. The research was a mixed-method using quantitative and qualitative methods with one group pretest-posttest design. There were 32 students of class XI MIA MA NW Wanasaba, Wanasaba Sub-District, East Lombok Regency as the sample. Instrument of this research was the mathematical reasoning abilities essay test consists of 10 questions on learning materials of limit function. The quantitative data analysis using descriptive statistics with paired sampel t-test. According to the results of data analysis, it was discovered that (). This indicated that there was a significant difference between pretest and posttest. Based on the percentage of the mathematical reasoning abilities test of students, the percentage of pretest was 63% of the students have the mathematical reasoning abilities with good category and posttest was 34% of the students have the mathematical reasoning abilities with good category. In addition, there were 84% of the students prefer the blended learning than the fully online learning, because it was effective to the mathematical reasoning abilities of students. Based on the results of the research it can be concluded that the fully online learning was no more effective than blended learning.