Much attention has been devoted to the study of the improved oil recovery (IOR) method(s). However it still remains a challenge to evaluate the reservoir wettability quantitatively using actual core samples. Contact angle is considered as one of the most common methods to measure the preferential affinity of reservoir rocks. The main objectives of this study are to investigate the influences of droplet volume, brine salinity, liquid saturating rocks, oil acid number, and temperature on rock wettability of carbonate reservoir rock using sessile droplet method. Sixteen runs are undertaken using oil droplet volumes of 10, 15, 20, and 25 milliliters with different brine salinities of zero, 50,000, 100,000, and 150,000 ppm of NaCl, respectively, to study the effects of droplet volume and salinity on contact angle. Three runs using three different crude oils having acid numbers of 0.374, 0.561, and 0.986 mg KOH/gm samples are performed to investigate the influence of acid number on contact angle. Three runs are carried out using brine, crude oil and polymer solutions to study the effect of type of liquid saturating rock on contact angle. Finally two runs are used to study the effect of temperature on contact angle. In all experiments, actual rock and crude oil samples are used. The results indicated that there is a specific droplet volume attained at critical water salinity. This critical water salinity is defined to be the salinity at which the oil droplet volume has no effect on contact angle. The rock wettability increases ( contact angle decreases) before the critical salinity and increases after it, depending upon the droplet volume. The attained results indicated that the liquid saturating rock has an important role on measured values of contact angle. The results showed that the increase of acid number of the crude oil decreases the contact angle. Therefore, carbonate oil reservoirs containing oils of low acid number are expected to be more oil-wet than ones containing oils of higher acid number. The increase of temperature and bath liquid viscosity reduced the measured contact angle. The results of this study developed a new concept of critical salinity and provided a better understanding of some factors affecting wettability measurements using contact angle technique.