The various embodiments herein provide a method for determining core wettability characteristics of reservoir rock samples based on modified form of Washburn equation. The method involves saturating a core sample with a first reservoir fluid such as water and imbibing with a second reservoir fluid such as oil. The change in square of the core mass with respect to time is monitored during an imbibition process to acquire a data to calculate a contact angle to determine the wettability of the sample. The contact angle is calculated by using the modified form of Washburn equation. A single or twin core sample is used to calculate the wettability characteristics. The method measures wettability characteristics in terms of contact angle and not in terms of wettability index.