We have studied the variation of the time profile of X-ray emission of solar flares that occurred during the second half of solar cycle 23 (SC 23) and for about the full solar cycle 24 (SC 24) (2002–2018). We define a new index, called the "ratio index" (Rf), for all X-ray solar flares. This index is defined as the ratio of the flare's rising time interval by its total duration period. According to the ratio index, the X-ray solar flares are classified into two types: (1) sudden flares [Rf <

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... ], and (2) gradual flares [Rf > 0.5]. The sudden flare type, with fast-rising and slow recovery, is more common and represents most of the flares that happen most of the time during the solar cycles but are less common during the minimum solar activity years. On the other hand, the gradual flare type (or Rf > 0.5) is less common but predominates during the minimum solar activity epochs. Sudden flares tend to be strong, large, and numerous in the polar regions, while gradual flares are weak, short, and countable in the latitude range between 50 and 70, both for northern and southern latitudes. However, both types appear to happen in the lower latitudes and the solar equatorial regions.